Stundenplan: Wochenend-Flakiness in Unterrichtsmodus-Test behoben

OpenWeekCell_LessonHeuteWaehrendUnterrichtszeit_OeffnetUnterrichtsmodus schlug
an Sa/So fehl, weil today.DayOfWeek dort keiner Kachel im Mo-Fr-Wochenraster
entspricht. TimetableViewModel bekommt dafür eine austauschbare Clock-Property
(Standard: DateTime.Now), sodass der Test "heute" unabhängig vom tatsächlichen
Wochentag simulieren kann.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -4,3 +4,6 @@
 # Mindestens 32 zufällige Zeichen – z.B. generiert mit:
 # openssl rand -base64 32
 JWT_SECRET=hier-einen-langen-zufaelligen-wert-eintragen
+
+# Identifiziert die Installation gegenüber Nominatim/DWD. Bei eigener Domain bitte anpassen.
+GEOCODING_USER_AGENT=LehrerApp-Server/1.0 (+https://example.org)

.gitignore

@@ -2,6 +2,7 @@
 bin/
 obj/
 out/
+build/
 
 # ── .NET ──────────────────────────────────────────────────────────────────────
 *.user

CLAUDE.md

@@ -60,12 +60,16 @@ The API can also run via `docker/docker-compose.yml` (reads `JWT_SECRET` from th
   orchestration, timer-driven), `SnapshotService`, `Crypto/SyncCrypto` (AES-256-GCM payload
   encryption — desktop events are encrypted at rest and in transit; Companion/WebApp events are
   plaintext, see `PlainSyncEvent` vs `SyncEvent`).
+- **LehrerApp.WebUntis** — direkter, serverunabhängiger WebUntis-Client für den Desktop. Hält die
+  persönliche JSON-RPC-Sitzung lokal und parst den Schülerreport lokal; WebUntis-Zugangsdaten und
+  personenbezogene Antworten dürfen nicht über `LehrerApp.Api` geleitet werden.
 - **LehrerApp.Api** — minimal ASP.NET Core server: JWT auth, an append-only `EventStore` plus
   `SnapshotStore`/`ReadableSnapshotStore` per device, mapped in `Endpoints/Endpoints.cs`. Sync is
   optional — Desktop only registers `SyncEngine`/`SnapshotService` in DI when a server URL is
   configured (`AppBootstrapper.LoadServerUrl`).
 - Each library has a matching `*.Tests` project (`LehrerApp.Tests` → Core, `LehrerApp.Data.Tests` →
   Data, `LehrerApp.Desktop.Tests` → Desktop, `LehrerApp.Sync.Tests` → Sync), all xUnit.
+  `LehrerApp.WebUntis.Tests` covers the direct WebUntis client and report parser.
 
 ### MVVM conventions (Desktop)
 

Directory.Packages.props

@@ -17,6 +17,10 @@
     <!-- Datenbank -->
     <PackageVersion Include="LiteDB" Version="5.0.21" />
 
+    <!-- PDF-Erzeugung (11.3) - nur LehrerApp.Desktop: zieht SkiaSharp-Native-Binaries mit,
+         die im Server-Image (LehrerApp.Api) nichts verloren haben. -->
+    <PackageVersion Include="QuestPDF" Version="2025.7.0" />
+
     <!-- API -->
     <PackageVersion Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.0" />
 

LehrerApp.Api.Tests/AssemblyInfo.cs

@@ -0,0 +1,10 @@
+using Xunit;
+
+// LiteDB nutzt einen statischen, geteilten BsonMapper.Global für die Reflection-basierte
+// Index-Auflösung (EnsureIndex mit Lambda-Ausdrücken). Bei paralleler Testausführung über
+// mehrere Testklassen hinweg (xUnit-Standard) konkurrieren mehrere Threads beim erstmaligen
+// Aufbau der Typ-Metadaten für verschiedene Modelle — das führt zu sporadischen
+// "Member X not found on BsonMapper"-Fehlern, die mit dem eigentlichen Testinhalt nichts zu
+// tun haben (gleiches Muster wie in LehrerApp.Data.Tests/LehrerApp.Desktop.Tests). Tests in
+// diesem Projekt laufen deshalb sequenziell.
+[assembly: CollectionBehavior(DisableTestParallelization = true)]

LehrerApp.Api.Tests/AttachmentStoreTests.cs

@@ -0,0 +1,71 @@
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class AttachmentStoreTests
+{
+    [Fact]
+    public async Task StoreAsync_GefolgtVonOpenRead_LiefertByteidentischeDatei()
+    {
+        using var temp = new TempDataPath();
+        var store = new AttachmentStore(temp.Path);
+        byte[] original = [1, 2, 3, 4, 5, 255, 0, 42];
+
+        await store.StoreAsync("user-1", "abc123", new MemoryStream(original));
+
+        using var read = store.OpenRead("user-1", "abc123");
+        Assert.NotNull(read);
+        using var ms = new MemoryStream();
+        await read!.CopyToAsync(ms);
+        Assert.Equal(original, ms.ToArray());
+    }
+
+    [Fact]
+    public void OpenRead_UnbekannteStorageId_GibtNullZurueck()
+    {
+        using var temp = new TempDataPath();
+        var store = new AttachmentStore(temp.Path);
+
+        Assert.Null(store.OpenRead("user-1", "unbekannt"));
+    }
+
+    [Fact]
+    public async Task StoreAsync_TrenntAnhaengeVerschiedenerNutzer()
+    {
+        using var temp = new TempDataPath();
+        var store = new AttachmentStore(temp.Path);
+
+        await store.StoreAsync("user-1", "shared-id", new MemoryStream([1]));
+
+        Assert.Null(store.OpenRead("user-2", "shared-id"));
+        using var user1Attachment = store.OpenRead("user-1", "shared-id");
+        Assert.NotNull(user1Attachment);
+    }
+
+    [Fact]
+    public async Task StoreAsync_BereinigtStorageIdMitPathTraversalZeichen()
+    {
+        using var temp = new TempDataPath();
+        var store = new AttachmentStore(temp.Path);
+
+        // Darf keinesfalls außerhalb von <root>/attachments/<user> landen.
+        await store.StoreAsync("user-1", "../../evil", new MemoryStream([1, 2, 3]));
+
+        Assert.False(File.Exists(Path.Combine(temp.Path, "evil")));
+        var withinRoot = Directory.EnumerateFiles(Path.Combine(temp.Path, "attachments"), "*", SearchOption.AllDirectories);
+        Assert.Contains(withinRoot, f => Path.GetFileName(f) == "evil");
+    }
+
+    private sealed class TempDataPath : IDisposable
+    {
+        public string Path { get; } = System.IO.Path.Combine(
+            System.IO.Path.GetTempPath(), $"lehrerapp-api-tests-attachments-{Guid.NewGuid():N}");
+
+        public TempDataPath() => Directory.CreateDirectory(Path);
+
+        public void Dispose()
+        {
+            if (Directory.Exists(Path)) Directory.Delete(Path, recursive: true);
+        }
+    }
+}

LehrerApp.Api.Tests/DwdWeatherServiceTests.cs

@@ -0,0 +1,118 @@
+using System.IO.Compression;
+using System.Text;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class DwdWeatherServiceTests
+{
+    [Fact]
+    public void ParseStationCatalog_LiestFestbreitenformatUndFindetNaechsteStation()
+    {
+        const string catalog = """
+            ID    ICAO NAME                 LAT    LON     ELEV
+            ----- ---- -------------------- -----  ------- -----
+            10641 EDDF OFFENBACH-WETTERPARK 50.08    8.78   119
+            10382 EDDT BERLIN-TEGEL         52.56   13.31    37
+            """;
+
+        var stations = DwdWeatherService.ParseStationCatalog(catalog);
+        var nearest = DwdWeatherService.FindNearestStation(stations, 50.1, 8.7);
+
+        Assert.Equal(2, stations.Count);
+        Assert.Equal("10641", nearest!.Id);
+        Assert.Equal("OFFENBACH-WETTERPARK", nearest.Name);
+    }
+
+    [Fact]
+    public void ParseMosmix_KonvertiertEinheitenUndBehaeltFehlwerteAlsNull()
+    {
+        var now = DateTime.UtcNow.AddHours(1).ToString("yyyy-MM-dd'T'HH:00:00.000'Z'");
+        var kml = $$"""
+            <?xml version="1.0" encoding="UTF-8"?>
+            <kml:kml xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:dwd="https://opendata.dwd.de/weather/lib/pointforecast_dwd_extension_V1_0.xsd">
+              <kml:Document><kml:ExtendedData><dwd:ProductDefinition>
+                <dwd:IssueTime>2026-08-23T15:00:00Z</dwd:IssueTime>
+                <dwd:ForecastTimeSteps><dwd:TimeStep>{{now}}</dwd:TimeStep></dwd:ForecastTimeSteps>
+              </dwd:ProductDefinition></kml:ExtendedData><kml:Placemark><kml:ExtendedData>
+                <dwd:Forecast dwd:elementName="TTT"><dwd:value>293.15</dwd:value></dwd:Forecast>
+                <dwd:Forecast dwd:elementName="FF"><dwd:value>5</dwd:value></dwd:Forecast>
+                <dwd:Forecast dwd:elementName="FX1"><dwd:value>-</dwd:value></dwd:Forecast>
+                <dwd:Forecast dwd:elementName="RR1c"><dwd:value>1.25</dwd:value></dwd:Forecast>
+                <dwd:Forecast dwd:elementName="R101"><dwd:value>67.6</dwd:value></dwd:Forecast>
+                <dwd:Forecast dwd:elementName="SunD1"><dwd:value>1800</dwd:value></dwd:Forecast>
+                <dwd:Forecast dwd:elementName="Neff"><dwd:value>75</dwd:value></dwd:Forecast>
+                <dwd:Forecast dwd:elementName="ww"><dwd:value>61</dwd:value></dwd:Forecast>
+              </kml:ExtendedData></kml:Placemark></kml:Document>
+            </kml:kml>
+            """;
+
+        var parsed = DwdWeatherService.ParseMosmix(Zip("forecast.kml", kml));
+        var hour = Assert.Single(parsed.Hours);
+
+        Assert.Equal(20, hour.TemperatureC);
+        Assert.Equal(18, hour.WindSpeedKmh);
+        Assert.Null(hour.WindGustKmh);
+        Assert.Equal(1.3, hour.PrecipitationMm);
+        Assert.Equal(68, hour.PrecipitationProbabilityPercent);
+        Assert.Equal(30, hour.SunshineDurationMinutes);
+        Assert.Equal(61, hour.WeatherCode);
+    }
+
+    [Fact]
+    public void ParseMosmix_LiefertMehrAlsDreiTageFuerFolgewoche()
+    {
+        var start = DateTime.UtcNow.AddHours(1);
+        var timeSteps = string.Join("", Enumerable.Range(0, 100)
+            .Select(i => $"<dwd:TimeStep>{start.AddHours(i):yyyy-MM-dd'T'HH:mm:ss'Z'}</dwd:TimeStep>"));
+        var values = string.Join(' ', Enumerable.Repeat("293.15", 100));
+        var kml = $$"""
+            <?xml version="1.0" encoding="UTF-8"?>
+            <kml:kml xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:dwd="https://opendata.dwd.de/weather/lib/pointforecast_dwd_extension_V1_0.xsd">
+              <kml:Document><kml:ExtendedData><dwd:ProductDefinition>
+                <dwd:ForecastTimeSteps>{{timeSteps}}</dwd:ForecastTimeSteps>
+              </dwd:ProductDefinition></kml:ExtendedData><kml:Placemark><kml:ExtendedData>
+                <dwd:Forecast dwd:elementName="TTT"><dwd:value>{{values}}</dwd:value></dwd:Forecast>
+              </kml:ExtendedData></kml:Placemark></kml:Document>
+            </kml:kml>
+            """;
+
+        var parsed = DwdWeatherService.ParseMosmix(Zip("forecast.kml", kml));
+
+        Assert.Equal(100, parsed.Hours.Count);
+    }
+
+    [Fact]
+    public void ParseCapArchive_OrdnetWarnpolygonGeografischZu()
+    {
+        var xml = """
+            <alert xmlns="urn:oasis:names:tc:emergency:cap:1.2">
+              <identifier>warnung-1</identifier><status>Actual</status>
+              <info><language>de-DE</language><event>GEWITTER</event><severity>Severe</severity>
+                <onset>2026-08-23T18:00:00Z</onset><expires>2099-08-23T20:00:00Z</expires>
+                <headline>Amtliche Warnung</headline><description>Test</description>
+                <area><polygon>50.0,8.0 50.0,9.0 51.0,9.0 51.0,8.0 50.0,8.0</polygon></area>
+              </info>
+            </alert>
+            """;
+
+        var warning = Assert.Single(DwdWeatherService.ParseCapArchive(
+            Zip("warning.xml", xml), DateTime.UtcNow));
+
+        Assert.True(DwdWeatherService.PointInPolygon(warning.Polygons[0], 50.5, 8.5));
+        Assert.False(DwdWeatherService.PointInPolygon(warning.Polygons[0], 52.0, 8.5));
+        Assert.Equal("GEWITTER", warning.Warning.Event);
+    }
+
+    private static byte[] Zip(string fileName, string content)
+    {
+        using var memory = new MemoryStream();
+        using (var archive = new ZipArchive(memory, ZipArchiveMode.Create, leaveOpen: true))
+        {
+            var entry = archive.CreateEntry(fileName);
+            using var writer = new StreamWriter(entry.Open(), new UTF8Encoding(false));
+            writer.Write(content);
+        }
+        return memory.ToArray();
+    }
+}

LehrerApp.Api.Tests/EventStoreTests.cs

@@ -0,0 +1,184 @@
+using LehrerApp.Sync.Models;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class EventStoreTests
+{
+    private static SyncEvent MakeEvent(string deviceId, string entityType = "Lesson", string operation = "Save",
+        string? entityId = null, long? basedOnServerSeq = null) => new()
+    {
+        DeviceId = deviceId, DeviceType = DeviceType.Desktop,
+        EntityType = entityType, EntityId = entityId ?? Guid.NewGuid().ToString(),
+        Operation = operation, Payload = "encrypted", BasedOnServerSeq = basedOnServerSeq,
+    };
+
+    [Fact]
+    public void Pull_KeineNeuenEreignisse_LiefertUnverändertesSinceAlsWatermark()
+    {
+        using var temp = new TempEventStore();
+        temp.Events.Push("user", [MakeEvent("device-a")]);
+
+        var result = temp.Events.Pull("user", since: 5, requestingDeviceId: "device-b");
+
+        Assert.Empty(result.Events);
+        Assert.Equal(5, result.ServerSequenceNr);
+    }
+
+    [Fact]
+    public void Pull_LiefertEreignisseAnderesGeraets_WatermarkEntsprichtHoechsterZurueckgegebenerServerSeq()
+    {
+        using var temp = new TempEventStore();
+        temp.Events.Push("user", [MakeEvent("device-a"), MakeEvent("device-a")]);
+
+        var result = temp.Events.Pull("user", since: 0, requestingDeviceId: "device-b");
+
+        Assert.Equal(2, result.Events.Count);
+        Assert.Equal(2, result.ServerSequenceNr);
+    }
+
+    /// Regression: das genau beobachtete Symptom — Gerät A pusht erfolgreich (Server bestätigt),
+    /// Gerät B pullt und bekommt 0 Ereignisse zurück, obwohl Gerät A's Ereignisse eigentlich für
+    /// Gerät B bestimmt waren. Ursache: Pull() gab bisher IMMER den globalen Höchststand
+    /// (LastSeq, inkl. der Ereignisse des ANFRAGENDEN Geräts selbst) als neuen "since"-Cursor
+    /// zurück — SyncEngine.PullAsync übernimmt den 1:1. Hat das anfragende Gerät SELBST kurz vorher
+    /// etwas gepusht (dessen Ereignisse hier bewusst per "DeviceId != requestingDeviceId"
+    /// herausgefiltert werden), sprang der Cursor über die noch gar nicht abgeholten Ereignisse
+    /// eines ANDEREN Geräts hinweg, wenn die eigenen neuer waren — sie wurden dauerhaft verpasst.
+    [Fact]
+    public void Pull_AnfragendesGeraetHatSelbstNeuereEreignisseGepusht_UeberspringtFremdeEreignisseNicht()
+    {
+        using var temp = new TempEventStore();
+        // Gerät A pusht zuerst (ServerSeq 1) - das ist die Änderung, die Gerät B eigentlich abholen soll.
+        temp.Events.Push("user", [MakeEvent("device-a")]);
+        // Gerät B hat selbst (z.B. im selben Sync-Zyklus, Push läuft immer vor Pull) etwas Neueres
+        // gepusht (ServerSeq 2) - dieses Ereignis gehört B selbst und wird unten aus der Pull-Antwort
+        // herausgefiltert.
+        temp.Events.Push("user", [MakeEvent("device-b")]);
+
+        // Gerät B pullt mit einem "since" von VOR seinem eigenen Push (0) - Geräts A's Ereignis
+        // (ServerSeq 1) muss zurückkommen.
+        var result = temp.Events.Pull("user", since: 0, requestingDeviceId: "device-b");
+
+        var evt = Assert.Single(result.Events);
+        Assert.Equal("device-a", evt.DeviceId);
+        // Der Bug: hier stand vorher 2 (globaler Höchststand, inkl. Geräts B eigenem Ereignis) -
+        // ein zweiter Pull mit since=2 hätte Geräts A's Ereignis (ServerSeq 1) nie wieder gefunden.
+        Assert.Equal(1, result.ServerSequenceNr);
+    }
+
+    // ── Exakte Kollisionsprüfung (BasedOnServerSeq statt 30s-Heuristik) ─────────
+
+    [Fact]
+    public void Push_NeueEntitaetOhneBasedOnServerSeq_WirdAngenommen()
+    {
+        using var temp = new TempEventStore();
+
+        var result = temp.Events.Push("user", [MakeEvent("device-a", basedOnServerSeq: null)]);
+
+        Assert.Empty(result.ConflictingEventIds);
+    }
+
+    [Fact]
+    public void Push_ZweitesEreignisMitFalschemBasedOnServerSeq_WirdAbgelehnt()
+    {
+        using var temp = new TempEventStore();
+        var entityId = Guid.NewGuid().ToString();
+        temp.Events.Push("user", [MakeEvent("device-a", entityId: entityId, basedOnServerSeq: null)]); // -> ServerSeq 1
+
+        // Gerät B kennt die Entität noch gar nicht (oder einen veralteten Stand) und behauptet fälschlich, sie sei neu.
+        var second = MakeEvent("device-b", entityId: entityId, basedOnServerSeq: null);
+        var result = temp.Events.Push("user", [second]);
+
+        Assert.Contains(second.EventId, result.ConflictingEventIds);
+    }
+
+    [Fact]
+    public void Push_ZweitesEreignisMitKorrektemBasedOnServerSeq_WirdAngenommen()
+    {
+        using var temp = new TempEventStore();
+        var entityId = Guid.NewGuid().ToString();
+        var first = temp.Events.Push("user", [MakeEvent("device-a", entityId: entityId, basedOnServerSeq: null)]);
+        var assignedSeq = first.AssignedServerSeqs.Values.Single(); // ServerSeq 1
+
+        var second = MakeEvent("device-b", entityId: entityId, basedOnServerSeq: assignedSeq);
+        var result = temp.Events.Push("user", [second]);
+
+        Assert.Empty(result.ConflictingEventIds);
+    }
+
+    [Fact]
+    public void Push_AkzeptierteEreignisse_LiefernJeEventIdDieVergebeneServerSeq()
+    {
+        using var temp = new TempEventStore();
+        var evt1 = MakeEvent("device-a");
+        var evt2 = MakeEvent("device-a");
+
+        var result = temp.Events.Push("user", [evt1, evt2]);
+
+        Assert.Equal(2, result.AssignedServerSeqs.Count);
+        Assert.True(result.AssignedServerSeqs.ContainsKey(evt1.EventId));
+        Assert.True(result.AssignedServerSeqs.ContainsKey(evt2.EventId));
+        Assert.Equal([1L, 2L], result.AssignedServerSeqs.Values.OrderBy(v => v));
+    }
+
+    [Fact]
+    public void Push_AbgelehntesEreignis_TauchtNichtInAssignedServerSeqsAuf()
+    {
+        using var temp = new TempEventStore();
+        var entityId = Guid.NewGuid().ToString();
+        temp.Events.Push("user", [MakeEvent("device-a", entityId: entityId, basedOnServerSeq: null)]);
+        var rejected = MakeEvent("device-b", entityId: entityId, basedOnServerSeq: null);
+
+        var result = temp.Events.Push("user", [rejected]);
+
+        Assert.False(result.AssignedServerSeqs.ContainsKey(rejected.EventId));
+    }
+
+    // ── GetLatestForEntity (Sofort-Nachladen nach abgelehntem Push) ─────────────
+
+    [Fact]
+    public void GetLatestForEntity_UnbekannteEntitaet_LiefertNull()
+    {
+        using var temp = new TempEventStore();
+
+        var result = temp.Events.GetLatestForEntity("user", "Lesson", Guid.NewGuid().ToString());
+
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void GetLatestForEntity_LiefertDenAktuellstenStandDieserEntitaet()
+    {
+        using var temp = new TempEventStore();
+        var entityId = Guid.NewGuid().ToString();
+        var first = temp.Events.Push("user", [MakeEvent("device-a", entityId: entityId, basedOnServerSeq: null)]);
+        var seq1 = first.AssignedServerSeqs.Values.Single();
+        temp.Events.Push("user", [MakeEvent("device-b", entityId: entityId, basedOnServerSeq: seq1)]);
+
+        var result = temp.Events.GetLatestForEntity("user", "Lesson", entityId);
+
+        Assert.NotNull(result);
+        Assert.Equal("device-b", result!.DeviceId);
+        Assert.Equal(seq1 + 1, result.SequenceNr);
+    }
+
+    private sealed class TempEventStore : IDisposable
+    {
+        private readonly string _directory = Path.Combine(
+            Path.GetTempPath(), $"lehrerapp-api-tests-eventstore-{Guid.NewGuid():N}");
+        public EventStore Events { get; }
+
+        public TempEventStore()
+        {
+            Directory.CreateDirectory(_directory);
+            Events = new EventStore(_directory);
+        }
+
+        public void Dispose()
+        {
+            Events.Dispose();
+            if (Directory.Exists(_directory)) Directory.Delete(_directory, recursive: true);
+        }
+    }
+}

LehrerApp.Api.Tests/LehrerApp.Api.Tests.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\LehrerApp.Api\LehrerApp.Api.csproj" />
+  </ItemGroup>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="xunit" />
+    <PackageReference Include="xunit.runner.visualstudio">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+  </ItemGroup>
+</Project>

LehrerApp.Api.Tests/NominatimGeocoderTests.cs

@@ -0,0 +1,46 @@
+using System.Net;
+using System.Text;
+using LehrerApp.Core.Models;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class NominatimGeocoderTests
+{
+    [Fact]
+    public async Task GeocodeAsync_NutztStrukturierteDeutschlandSucheUndLiestDisplayName()
+    {
+        Uri? requestedUri = null;
+        var handler = new RecordingHandler(request =>
+        {
+            requestedUri = request.RequestUri;
+            return new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new StringContent(
+                    """[{"lat":"50.105","lon":"8.761","display_name":"Testschule, Offenbach, Deutschland"}]""",
+                    Encoding.UTF8, "application/json"),
+            };
+        });
+        var http = new HttpClient(handler) { BaseAddress = new Uri("https://nominatim.openstreetmap.org/") };
+        var geocoder = new NominatimGeocoder(http);
+
+        var result = await geocoder.GeocodeAsync(new SchoolLocationRequest
+        {
+            SchoolName = "Testschule", Street = "Schulweg 1", PostalCode = "63000", City = "Offenbach",
+        }, CancellationToken.None);
+
+        Assert.NotNull(result);
+        Assert.Equal(50.105, result!.Latitude);
+        Assert.Equal("Testschule, Offenbach, Deutschland", result.DisplayName);
+        Assert.Contains("countrycodes=de", requestedUri!.Query);
+        Assert.Contains("street=Schulweg%201", requestedUri.Query);
+        Assert.Contains("postalcode=63000", requestedUri.Query);
+    }
+
+    private sealed class RecordingHandler(Func<HttpRequestMessage, HttpResponseMessage> response)
+        : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
+            CancellationToken cancellationToken) => Task.FromResult(response(request));
+    }
+}

LehrerApp.Api.Tests/PasswordHasherTests.cs

@@ -0,0 +1,41 @@
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class PasswordHasherTests
+{
+    [Fact]
+    public void Verify_RichtigesPasswort_GibtTrueZurueck()
+    {
+        var hash = PasswordHasher.Hash("korrektes-passwort-123");
+
+        Assert.True(PasswordHasher.Verify("korrektes-passwort-123", hash));
+    }
+
+    [Fact]
+    public void Verify_FalschesPasswort_GibtFalseZurueck()
+    {
+        var hash = PasswordHasher.Hash("korrektes-passwort-123");
+
+        Assert.False(PasswordHasher.Verify("falsches-passwort-456", hash));
+    }
+
+    [Fact]
+    public void Hash_ZweiAufrufeMitGleichemPasswort_ErzeugenUnterschiedlicheHashes()
+    {
+        // Zufälliger Salt pro Nutzer -> selbst identische Passwörter dürfen nicht denselben
+        // gespeicherten Wert ergeben.
+        var hash1 = PasswordHasher.Hash("dasselbe-passwort");
+        var hash2 = PasswordHasher.Hash("dasselbe-passwort");
+
+        Assert.NotEqual(hash1, hash2);
+        Assert.True(PasswordHasher.Verify("dasselbe-passwort", hash1));
+        Assert.True(PasswordHasher.Verify("dasselbe-passwort", hash2));
+    }
+
+    [Fact]
+    public void Verify_UngueltigesGespeichertesFormat_GibtFalseZurueckStattZuWerfen()
+    {
+        Assert.False(PasswordHasher.Verify("irgendwas", "kein-gueltiges-format"));
+    }
+}

LehrerApp.Api.Tests/PlainEventStoreTests.cs

@@ -0,0 +1,54 @@
+using LehrerApp.Sync.Models;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class PlainEventStoreTests
+{
+    [Theory]
+    [InlineData("Grade")]
+    [InlineData("ExamResult")]
+    public void Push_PersonenbezogenerEntityTyp_WirdAbgelehnt(string entityType)
+    {
+        using var temp = new TempEventStore();
+        var evt = new PlainSyncEvent { EntityType = entityType, EntityId = "1", Operation = "Save" };
+
+        var result = temp.Plain.Push("user", [evt]);
+
+        Assert.Contains(evt.EventId, result.RejectedEventIds);
+    }
+
+    [Theory]
+    [InlineData("WorkTask")]
+    [InlineData("Lesson")]
+    public void Push_UnkritischerEntityTyp_WirdAngenommen(string entityType)
+    {
+        using var temp = new TempEventStore();
+        var evt = new PlainSyncEvent { EntityType = entityType, EntityId = "1", Operation = "Save" };
+
+        var result = temp.Plain.Push("user", [evt]);
+
+        Assert.DoesNotContain(evt.EventId, result.RejectedEventIds);
+    }
+
+    private sealed class TempEventStore : IDisposable
+    {
+        private readonly string _directory = Path.Combine(
+            Path.GetTempPath(), $"lehrerapp-api-tests-{Guid.NewGuid():N}");
+        private readonly EventStore _events;
+        public PlainEventStore Plain { get; }
+
+        public TempEventStore()
+        {
+            Directory.CreateDirectory(_directory);
+            _events = new EventStore(_directory);
+            Plain = new PlainEventStore(_events);
+        }
+
+        public void Dispose()
+        {
+            _events.Dispose();
+            if (Directory.Exists(_directory)) Directory.Delete(_directory, recursive: true);
+        }
+    }
+}

LehrerApp.Api.Tests/SchoolWeatherStoreTests.cs

@@ -0,0 +1,49 @@
+using LehrerApp.Core.Models;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class SchoolWeatherStoreTests
+{
+    [Fact]
+    public void SaveLocation_TrenntNutzerUndErsetztVorhandenenStandort()
+    {
+        using var temp = new TempStore();
+        temp.Store.SaveLocation("eins", Location("Köln", 50.9, 6.9));
+        temp.Store.SaveLocation("zwei", Location("Berlin", 52.5, 13.4));
+        temp.Store.SaveLocation("eins", Location("Bonn", 50.7, 7.1));
+
+        Assert.Equal("Bonn", temp.Store.GetLocation("eins")!.City);
+        Assert.Equal("Berlin", temp.Store.GetLocation("zwei")!.City);
+    }
+
+    [Fact]
+    public void SaveLocation_AenderungEntferntWettercacheDesAltenOrts()
+    {
+        using var temp = new TempStore();
+        temp.Store.SaveLocation("eins", Location("Köln", 50.9, 6.9));
+        temp.Store.SaveWeather("eins", new WeatherSnapshot { StationId = "alt", RetrievedAt = DateTime.UtcNow });
+
+        temp.Store.SaveLocation("eins", Location("Bonn", 50.7, 7.1));
+
+        Assert.Null(temp.Store.GetWeather("eins"));
+    }
+
+    private static SchoolLocationProfile Location(string city, double lat, double lon) => new()
+    {
+        SchoolName = "Testschule", Street = "Schulweg 1", PostalCode = "12345", City = city,
+        Latitude = lat, Longitude = lon, ResolvedAddress = city, UpdatedAt = DateTime.UtcNow,
+    };
+
+    private sealed class TempStore : IDisposable
+    {
+        private readonly string _path = Path.Combine(Path.GetTempPath(), $"lehrerapp-weather-tests-{Guid.NewGuid():N}");
+        public SchoolWeatherStore Store { get; }
+        public TempStore() { Directory.CreateDirectory(_path); Store = new(_path); }
+        public void Dispose()
+        {
+            Store.Dispose();
+            if (Directory.Exists(_path)) Directory.Delete(_path, recursive: true);
+        }
+    }
+}

LehrerApp.Api.Tests/SnapshotStoreTests.cs

@@ -0,0 +1,89 @@
+using LehrerApp.Sync.Models;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class SnapshotStoreTests
+{
+    [Fact]
+    public void Store_ZweiterAufrufOhneCode_ErzeugtNeuenCode()
+    {
+        using var temp = new TempSnapshotStore();
+
+        var first = temp.Store.Store("user1", new SnapshotUploadRequest { EncryptedPayload = "p1" });
+        var second = temp.Store.Store("user1", new SnapshotUploadRequest { EncryptedPayload = "p2" });
+
+        Assert.NotEqual(first.Code, second.Code);
+    }
+
+    /// Regression: SnapshotService.CreateAndUploadAsync lädt in zwei Schritten hoch — Schritt 1
+    /// ohne EncryptedSyncKey (holt einen Code vom Server), Schritt 2 mit dem gerade erst mit
+    /// diesem Code verschlüsselten Schlüssel. Ohne den zurückgereichten Code erzeugte Store() bei
+    /// jedem Aufruf einen NEUEN Zufallscode — der an den Nutzer angezeigte Code (aus Schritt 2)
+    /// passte dann nie zu dem Code, mit dem der Schlüssel tatsächlich verschlüsselt wurde, und das
+    /// Einlösen auf dem zweiten Gerät schlug beim Schlüssel-Entschlüsseln immer fehl.
+    [Fact]
+    public void Store_ZweiterAufrufMitCodeDesErstenSchritts_AktualisiertDenselbenEintrag()
+    {
+        using var temp = new TempSnapshotStore();
+
+        var step1 = temp.Store.Store("user1", new SnapshotUploadRequest { EncryptedPayload = "payload" });
+        var step2 = temp.Store.Store("user1", new SnapshotUploadRequest
+        {
+            EncryptedPayload = "payload", EncryptedSyncKey = "encrypted-key", Code = step1.Code,
+        });
+
+        Assert.Equal(step1.Code, step2.Code);
+
+        var retrieved = temp.Store.Retrieve("user1", step2.Code);
+        Assert.NotNull(retrieved);
+        Assert.Equal("encrypted-key", retrieved!.EncryptedSyncKey);
+    }
+
+    [Fact]
+    public void Store_CodeGehoertZuAnderemNutzer_ErzeugtStattdessenNeuenEintrag()
+    {
+        using var temp = new TempSnapshotStore();
+        var step1 = temp.Store.Store("user1", new SnapshotUploadRequest { EncryptedPayload = "payload" });
+
+        var result = temp.Store.Store("user2", new SnapshotUploadRequest
+        {
+            EncryptedPayload = "payload", EncryptedSyncKey = "encrypted-key", Code = step1.Code,
+        });
+
+        Assert.NotEqual(step1.Code, result.Code);
+        Assert.NotNull(temp.Store.Retrieve("user2", result.Code));
+    }
+
+    [Fact]
+    public void Retrieve_LoeschtDenEintragNachDemErstenAbruf()
+    {
+        using var temp = new TempSnapshotStore();
+        var uploaded = temp.Store.Store("user1", new SnapshotUploadRequest { EncryptedPayload = "payload" });
+
+        var first = temp.Store.Retrieve("user1", uploaded.Code);
+        var second = temp.Store.Retrieve("user1", uploaded.Code);
+
+        Assert.NotNull(first);
+        Assert.Null(second);
+    }
+
+    private sealed class TempSnapshotStore : IDisposable
+    {
+        private readonly string _directory = Path.Combine(
+            Path.GetTempPath(), $"lehrerapp-api-tests-{Guid.NewGuid():N}");
+        public LehrerApp.Api.SnapshotStore Store { get; }
+
+        public TempSnapshotStore()
+        {
+            Directory.CreateDirectory(_directory);
+            Store = new LehrerApp.Api.SnapshotStore(_directory);
+        }
+
+        public void Dispose()
+        {
+            Store.Dispose();
+            if (Directory.Exists(_directory)) Directory.Delete(_directory, recursive: true);
+        }
+    }
+}

LehrerApp.Api.Tests/SyncProtocolVersionMiddlewareTests.cs

@@ -0,0 +1,81 @@
+using LehrerApp.Sync;
+using Microsoft.AspNetCore.Http;
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class SyncProtocolVersionMiddlewareTests
+{
+    [Theory]
+    [InlineData("/api/sync/push")]
+    [InlineData("/api/sync/plain/push")]
+    [InlineData("/api/sync/attachments/datei")]
+    [InlineData("/api/snapshot/upload")]
+    public async Task FehlendeVersion_BlockiertAlleSyncKanaeleVorDemEndpoint(string path)
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context(path);
+
+        await middleware.InvokeAsync(context);
+
+        Assert.False(endpointReached);
+        Assert.Equal(StatusCodes.Status426UpgradeRequired, context.Response.StatusCode);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            context.Response.Headers[SyncProtocol.VersionHeaderName].ToString());
+    }
+
+    [Fact]
+    public async Task AbweichendeVersion_BlockiertRequest()
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context("/api/sync/push");
+        context.Request.Headers[SyncProtocol.VersionHeaderName] = "0";
+
+        await middleware.InvokeAsync(context);
+
+        Assert.False(endpointReached);
+        Assert.Equal(StatusCodes.Status426UpgradeRequired, context.Response.StatusCode);
+    }
+
+    [Fact]
+    public async Task AktuelleVersion_LaesstRequestZumEndpointDurch()
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context("/api/sync/push");
+        context.Request.Headers[SyncProtocol.VersionHeaderName] = SyncProtocol.CurrentVersion;
+
+        await middleware.InvokeAsync(context);
+
+        Assert.True(endpointReached);
+        Assert.NotEqual(StatusCodes.Status426UpgradeRequired, context.Response.StatusCode);
+    }
+
+    [Fact]
+    public async Task NichtSyncEndpoint_BrauchtKeineVersion()
+    {
+        var endpointReached = false;
+        var middleware = Middleware(() => endpointReached = true);
+        var context = Context("/api/auth/login");
+
+        await middleware.InvokeAsync(context);
+
+        Assert.True(endpointReached);
+    }
+
+    private static SyncProtocolVersionMiddleware Middleware(Action reached) => new(_ =>
+    {
+        reached();
+        return Task.CompletedTask;
+    });
+
+    private static DefaultHttpContext Context(string path)
+    {
+        var context = new DefaultHttpContext();
+        context.Request.Path = path;
+        context.Response.Body = new MemoryStream();
+        return context;
+    }
+}

LehrerApp.Api.Tests/UserStoreTests.cs

@@ -0,0 +1,105 @@
+using Xunit;
+
+namespace LehrerApp.Api.Tests;
+
+public sealed class UserStoreTests
+{
+    [Fact]
+    public void CreateUser_NeuerNutzername_GibtTrueZurueckUndKannSichAnmelden()
+    {
+        using var temp = new TempUserStore();
+
+        var created = temp.Store.CreateUser("sebastian", "einSicheresPasswort");
+
+        Assert.True(created);
+        Assert.NotNull(temp.Store.Authenticate("sebastian", "einSicheresPasswort"));
+    }
+
+    [Fact]
+    public void CreateUser_BereitsVorhandenerNutzername_GibtFalseZurueck()
+    {
+        using var temp = new TempUserStore();
+        temp.Store.CreateUser("sebastian", "einSicheresPasswort");
+
+        var created = temp.Store.CreateUser("sebastian", "einAnderesPasswort");
+
+        Assert.False(created);
+    }
+
+    [Fact]
+    public void Authenticate_FalschesPasswort_LiefertNull()
+    {
+        using var temp = new TempUserStore();
+        temp.Store.CreateUser("sebastian", "einSicheresPasswort");
+
+        Assert.Null(temp.Store.Authenticate("sebastian", "falschesPasswort"));
+    }
+
+    [Fact]
+    public void Authenticate_UnbekannterNutzer_LiefertNull()
+    {
+        using var temp = new TempUserStore();
+
+        Assert.Null(temp.Store.Authenticate("unbekannt", "irgendwas"));
+    }
+
+    /// Regression: LiteDBs Standard-Collation vergleicht den eindeutigen Index auf Username
+    /// case-insensitive - ein Login mit abweichender Groß-/Kleinschreibung authentifiziert also
+    /// erfolgreich. Würde der Aufrufer (Endpoints./api/auth/login) daraufhin den roh eingegebenen
+    /// Namen statt des kanonisch gespeicherten als userId für den Server-seitigen Event-Speicher
+    /// verwenden, würde ein einziger andersartig getippter Login auf einem zweiten Gerät einen
+    /// komplett getrennten, nicht überlappenden Datenbestand erzeugen (siehe TODO 10.2.5 - genau
+    /// das vom Nutzer beobachtete Symptom: nur ein Konto im System, aber Client 2 erhielt trotz
+    /// since=0 keine Ereignisse).
+    [Fact]
+    public void Authenticate_AndereGrossKleinschreibung_LiefertDenKanonischGespeichertenNamen()
+    {
+        using var temp = new TempUserStore();
+        temp.Store.CreateUser("sebastian", "einSicheresPasswort");
+
+        var user = temp.Store.Authenticate("Sebastian", "einSicheresPasswort");
+
+        Assert.NotNull(user);
+        Assert.Equal("sebastian", user!.Username);
+    }
+
+    [Fact]
+    public void SetPassword_VorhandenerNutzer_AktualisiertPasswortUndAltesFunktioniertNichtMehr()
+    {
+        using var temp = new TempUserStore();
+        temp.Store.CreateUser("sebastian", "altesPasswort123");
+
+        var updated = temp.Store.SetPassword("sebastian", "neuesPasswort123");
+
+        Assert.True(updated);
+        Assert.NotNull(temp.Store.Authenticate("sebastian", "neuesPasswort123"));
+        Assert.Null(temp.Store.Authenticate("sebastian", "altesPasswort123"));
+    }
+
+    [Fact]
+    public void SetPassword_UnbekannterNutzer_GibtFalseZurueck()
+    {
+        using var temp = new TempUserStore();
+
+        Assert.False(temp.Store.SetPassword("unbekannt", "irgendeinPasswort"));
+    }
+
+    private sealed class TempUserStore : IDisposable
+    {
+        private readonly string _directory = Path.Combine(
+            Path.GetTempPath(), $"lehrerapp-api-tests-{Guid.NewGuid():N}");
+        public UserStore Store { get; }
+
+        public TempUserStore()
+        {
+            Directory.CreateDirectory(_directory);
+            Store = new UserStore(_directory);
+        }
+
+        public void Dispose()
+        {
+            Store.Dispose();
+            if (Directory.Exists(_directory)) Directory.Delete(_directory, recursive: true);
+        }
+    }
+}

LehrerApp.Api/AttachmentStore.cs

@@ -0,0 +1,29 @@
+namespace LehrerApp.Api;
+
+/// <summary>
+/// Rohdaten-Ablage für verschlüsselte Datei-Anhänge, getrennt vom Ereignis-/Snapshot-Speicher.
+/// Server sieht nur verschlüsselte Bytes, kein LiteDB nötig - einfache Dateien pro Nutzer/Id.
+/// </summary>
+public class AttachmentStore(string dataPath)
+{
+    private readonly string _root = Path.Combine(dataPath, "attachments");
+
+    public async Task StoreAsync(string userId, string storageId, Stream content)
+    {
+        var dir = Path.Combine(_root, Safe(userId));
+        Directory.CreateDirectory(dir);
+        await using var file = File.Create(Path.Combine(dir, Safe(storageId)));
+        await content.CopyToAsync(file);
+    }
+
+    public Stream? OpenRead(string userId, string storageId)
+    {
+        var path = Path.Combine(_root, Safe(userId), Safe(storageId));
+        return File.Exists(path) ? File.OpenRead(path) : null;
+    }
+
+    // storageId kommt als Routen-Parameter vom Client - nie ungeprüft in einen Dateipfad
+    // übernehmen (Path-Traversal).
+    private static string Safe(string value) =>
+        string.Concat(value.Where(c => char.IsLetterOrDigit(c) || c == '-'));
+}

LehrerApp.Api/Cli.cs

@@ -0,0 +1,85 @@
+using System.Text;
+
+namespace LehrerApp.Api;
+
+internal static class Cli
+{
+    public static Task<int> RunCreateUserAsync(string dataPath, string[] args)
+    {
+        if (args.Length < 2)
+        {
+            Console.Error.WriteLine("Verwendung: create-user <benutzername> [--password <passwort>]");
+            return Task.FromResult(1);
+        }
+        var username = args[1];
+        var password = ParsePassword(args);
+        if (password is null) return Task.FromResult(1);
+
+        Directory.CreateDirectory(dataPath);
+        using var store = new UserStore(dataPath);
+        if (!store.CreateUser(username, password))
+        {
+            Console.Error.WriteLine($"Nutzer '{username}' existiert bereits.");
+            return Task.FromResult(1);
+        }
+
+        Console.WriteLine($"Nutzer '{username}' angelegt.");
+        return Task.FromResult(0);
+    }
+
+    public static Task<int> RunSetPasswordAsync(string dataPath, string[] args)
+    {
+        if (args.Length < 2)
+        {
+            Console.Error.WriteLine("Verwendung: set-password <benutzername> [--password <passwort>]");
+            return Task.FromResult(1);
+        }
+        var username = args[1];
+        var password = ParsePassword(args);
+        if (password is null) return Task.FromResult(1);
+
+        Directory.CreateDirectory(dataPath);
+        using var store = new UserStore(dataPath);
+        if (!store.SetPassword(username, password))
+        {
+            Console.Error.WriteLine($"Nutzer '{username}' existiert nicht.");
+            return Task.FromResult(1);
+        }
+
+        Console.WriteLine($"Passwort für '{username}' aktualisiert.");
+        return Task.FromResult(0);
+    }
+
+    private static string? ParsePassword(string[] args)
+    {
+        string? password = null;
+        for (var i = 2; i < args.Length - 1; i++)
+            if (args[i] == "--password") password = args[i + 1];
+
+        password ??= ReadPassword("Passwort (mind. 12 Zeichen): ");
+
+        if (string.IsNullOrWhiteSpace(password) || password.Length < 12)
+        {
+            Console.Error.WriteLine("Passwort muss mindestens 12 Zeichen lang sein.");
+            return null;
+        }
+        return password;
+    }
+
+    // docker exec ohne -it liefert kein TTY -> ReadKey wäre nicht möglich, dann normal lesen.
+    private static string ReadPassword(string prompt)
+    {
+        Console.Write(prompt);
+        if (Console.IsInputRedirected) return Console.ReadLine() ?? "";
+
+        var sb = new StringBuilder();
+        ConsoleKeyInfo key;
+        while ((key = Console.ReadKey(intercept: true)).Key != ConsoleKey.Enter)
+        {
+            if (key.Key == ConsoleKey.Backspace && sb.Length > 0) { sb.Length--; Console.Write("\b \b"); }
+            else if (!char.IsControl(key.KeyChar)) { sb.Append(key.KeyChar); Console.Write('*'); }
+        }
+        Console.WriteLine();
+        return sb.ToString();
+    }
+}

LehrerApp.Api/DwdWeatherService.cs

@@ -0,0 +1,272 @@
+using System.Globalization;
+using System.IO.Compression;
+using System.Text.RegularExpressions;
+using System.Xml.Linq;
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Api;
+
+/// <summary>Lädt DWD-MOSMIX-Punktvorhersagen und CAP-Warnungen. Alle externen Antworten werden
+/// pro Serverinstanz gecacht; pro Benutzer wird zusätzlich der letzte erfolgreiche Snapshot im
+/// SchoolWeatherStore persistiert.</summary>
+public sealed class DwdWeatherService(HttpClient http)
+{
+    private const string StationCatalogUrl =
+        "https://www.dwd.de/DE/leistungen/met_verfahren_mosmix/mosmix_stationskatalog.cfg?view=nasPublication";
+    private const string ForecastUrl =
+        "https://opendata.dwd.de/weather/local_forecasts/mos/MOSMIX_L/single_stations/{0}/kml/MOSMIX_L_LATEST_{0}.kmz";
+    private const string WarningsUrl =
+        "https://opendata.dwd.de/weather/alerts/cap/COMMUNEUNION_DWD_STAT/Z_CAP_C_EDZW_LATEST_PVW_STATUS_PREMIUMDWD_COMMUNEUNION_DE.zip";
+
+    private readonly SemaphoreSlim _gate = new(1, 1);
+    private List<MosmixStation>? _stations;
+    private DateTime _stationsLoadedAt;
+    private readonly Dictionary<string, CachedForecast> _forecastCache = [];
+    private CachedWarnings? _warningsCache;
+
+    public async Task<WeatherSnapshot> GetAsync(SchoolLocationProfile location,
+        CancellationToken cancellationToken)
+    {
+        await _gate.WaitAsync(cancellationToken);
+        try
+        {
+            var stations = await GetStationsAsync(cancellationToken);
+            var station = FindNearestStation(stations, location.Latitude, location.Longitude)
+                ?? throw new InvalidOperationException("Der DWD-Stationskatalog enthält keine Stationen.");
+            var forecast = await GetForecastAsync(station, cancellationToken);
+            var warnings = await GetWarningsAsync(cancellationToken);
+
+            return new WeatherSnapshot
+            {
+                StationId = station.Id,
+                StationName = station.Name,
+                StationDistanceKm = Math.Round(DistanceKm(
+                    location.Latitude, location.Longitude, station.Latitude, station.Longitude), 1),
+                ForecastIssuedAt = forecast.IssuedAt,
+                RetrievedAt = DateTime.UtcNow,
+                Forecast = forecast.Hours,
+                Warnings = warnings.Where(w => Contains(w.Polygons, location.Latitude, location.Longitude))
+                                   .Select(w => w.Warning)
+                                   .OrderByDescending(w => SeverityRank(w.Severity))
+                                   .ThenBy(w => w.Onset)
+                                   .ToList(),
+            };
+        }
+        finally { _gate.Release(); }
+    }
+
+    private async Task<List<MosmixStation>> GetStationsAsync(CancellationToken cancellationToken)
+    {
+        if (_stations is not null && DateTime.UtcNow - _stationsLoadedAt < TimeSpan.FromDays(7))
+            return _stations;
+        var text = await http.GetStringAsync(StationCatalogUrl, cancellationToken);
+        _stations = ParseStationCatalog(text);
+        _stationsLoadedAt = DateTime.UtcNow;
+        return _stations;
+    }
+
+    private async Task<ParsedForecast> GetForecastAsync(MosmixStation station,
+        CancellationToken cancellationToken)
+    {
+        if (_forecastCache.TryGetValue(station.Id, out var cached) &&
+            DateTime.UtcNow - cached.LoadedAt < TimeSpan.FromMinutes(45))
+            return cached.Forecast;
+
+        var bytes = await http.GetByteArrayAsync(string.Format(CultureInfo.InvariantCulture,
+            ForecastUrl, station.Id), cancellationToken);
+        var parsed = ParseMosmix(bytes);
+        _forecastCache[station.Id] = new(DateTime.UtcNow, parsed);
+        return parsed;
+    }
+
+    private async Task<List<ParsedWarning>> GetWarningsAsync(CancellationToken cancellationToken)
+    {
+        if (_warningsCache is not null &&
+            DateTime.UtcNow - _warningsCache.LoadedAt < TimeSpan.FromMinutes(5))
+            return _warningsCache.Warnings;
+        var bytes = await http.GetByteArrayAsync(WarningsUrl, cancellationToken);
+        var warnings = ParseCapArchive(bytes, DateTime.UtcNow);
+        _warningsCache = new(DateTime.UtcNow, warnings);
+        return warnings;
+    }
+
+    public static List<MosmixStation> ParseStationCatalog(string text)
+    {
+        var regex = new Regex(@"^(\S+)\s+\S+\s+(.{20})\s+(-?\d+(?:\.\d+)?)\s+(-?\d+(?:\.\d+)?)\s+", RegexOptions.Compiled);
+        var result = new List<MosmixStation>();
+        foreach (var line in text.Split('\n'))
+        {
+            var match = regex.Match(line.TrimEnd('\r'));
+            if (!match.Success ||
+                !double.TryParse(match.Groups[3].Value, NumberStyles.Float, CultureInfo.InvariantCulture, out var lat) ||
+                !double.TryParse(match.Groups[4].Value, NumberStyles.Float, CultureInfo.InvariantCulture, out var lon))
+                continue;
+            result.Add(new(match.Groups[1].Value, match.Groups[2].Value.Trim(), lat, lon));
+        }
+        return result;
+    }
+
+    public static MosmixStation? FindNearestStation(IEnumerable<MosmixStation> stations,
+        double latitude, double longitude) => stations.MinBy(s =>
+            DistanceKm(latitude, longitude, s.Latitude, s.Longitude));
+
+    public static ParsedForecast ParseMosmix(byte[] kmz)
+    {
+        using var memory = new MemoryStream(kmz);
+        using var archive = new ZipArchive(memory, ZipArchiveMode.Read);
+        var entry = archive.Entries.FirstOrDefault(x => x.Name.EndsWith(".kml", StringComparison.OrdinalIgnoreCase))
+            ?? throw new InvalidDataException("Die MOSMIX-Datei enthält kein KML-Dokument.");
+        using var stream = entry.Open();
+        var document = XDocument.Load(stream);
+        XNamespace dwd = "https://opendata.dwd.de/weather/lib/pointforecast_dwd_extension_V1_0.xsd";
+
+        var times = document.Descendants(dwd + "TimeStep")
+            .Select(x => ParseDate(x.Value))
+            .Where(x => x.HasValue)
+            .Select(x => x!.Value)
+            .ToList();
+        if (times.Count == 0) throw new InvalidDataException("MOSMIX enthält keine Zeitschritte.");
+
+        var values = document.Descendants(dwd + "Forecast")
+            .Where(x => x.Attribute(dwd + "elementName") is not null)
+            .ToDictionary(
+                x => x.Attribute(dwd + "elementName")!.Value,
+                x => ParseValues(x.Element(dwd + "value")?.Value ?? ""),
+                StringComparer.Ordinal);
+
+        double? Value(string key, int i) => values.TryGetValue(key, out var series) && i < series.Count
+            ? series[i] : null;
+        var hours = new List<WeatherForecastHour>();
+        var earliest = DateTime.UtcNow.AddHours(-1);
+        // MOSMIX_L reicht bis +240 h. Die vollständige Spanne ist für die Unterrichtsplanung
+        // sinnvoll, weil das Wochenraster auch in die Folgewoche geblättert werden kann.
+        for (var i = 0; i < times.Count && hours.Count < 240; i++)
+        {
+            if (times[i] < earliest) continue;
+            hours.Add(new WeatherForecastHour
+            {
+                ValidAt = times[i],
+                TemperatureC = Round(Value("TTT", i) - 273.15, 1),
+                WindSpeedKmh = Round(Value("FF", i) * 3.6, 1),
+                WindGustKmh = Round(Value("FX1", i) * 3.6, 1),
+                PrecipitationMm = Round(Value("RR1c", i), 1),
+                // Beide Zusatzwerte stammen aus derselben MOSMIX-L-Datei. R101 bezeichnet die
+                // Wahrscheinlichkeit für >0,1 mm in der letzten Stunde, SunD1 liegt in Sekunden vor.
+                PrecipitationProbabilityPercent = Round(Value("R101", i), 0),
+                SunshineDurationMinutes = Round(Value("SunD1", i) / 60, 0),
+                CloudCoverPercent = Round(Value("Neff", i), 0),
+                WeatherCode = Value("ww", i) is { } code ? (int)Math.Round(code) : null,
+            });
+        }
+
+        return new(ParseDate(document.Descendants(dwd + "IssueTime").FirstOrDefault()?.Value), hours);
+    }
+
+    public static List<ParsedWarning> ParseCapArchive(byte[] zip, DateTime nowUtc)
+    {
+        var result = new List<ParsedWarning>();
+        using var memory = new MemoryStream(zip);
+        using var archive = new ZipArchive(memory, ZipArchiveMode.Read);
+        foreach (var entry in archive.Entries.Where(x => x.Name.EndsWith(".xml", StringComparison.OrdinalIgnoreCase)))
+        {
+            using var stream = entry.Open();
+            var document = XDocument.Load(stream);
+            XNamespace cap = "urn:oasis:names:tc:emergency:cap:1.2";
+            var root = document.Root;
+            if (root is null || root.Element(cap + "status")?.Value != "Actual") continue;
+            var info = root.Elements(cap + "info").FirstOrDefault(x =>
+                x.Element(cap + "language")?.Value.StartsWith("de", StringComparison.OrdinalIgnoreCase) == true)
+                ?? root.Elements(cap + "info").FirstOrDefault();
+            if (info is null) continue;
+            var expires = ParseDate(info.Element(cap + "expires")?.Value);
+            if (expires is { } until && until < nowUtc) continue;
+
+            var polygons = info.Elements(cap + "area")
+                .SelectMany(x => x.Elements(cap + "polygon"))
+                .Select(x => ParsePolygon(x.Value))
+                .Where(x => x.Count >= 3)
+                .ToList();
+            // Warngebiete ohne Polygon (z.B. Seegebiete) können einer Schulkoordinate nicht
+            // zuverlässig zugeordnet werden und werden daher bewusst nicht global angezeigt.
+            if (polygons.Count == 0) continue;
+            result.Add(new ParsedWarning(new WeatherWarning
+            {
+                Identifier = root.Element(cap + "identifier")?.Value ?? "",
+                Event = info.Element(cap + "event")?.Value ?? "",
+                Headline = info.Element(cap + "headline")?.Value ?? "",
+                Description = info.Element(cap + "description")?.Value ?? "",
+                Instruction = info.Element(cap + "instruction")?.Value ?? "",
+                Severity = info.Element(cap + "severity")?.Value ?? "Unknown",
+                Onset = ParseDate(info.Element(cap + "onset")?.Value),
+                Expires = expires,
+            }, polygons));
+        }
+        return result;
+    }
+
+    private static List<double?> ParseValues(string raw) => raw
+        .Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries)
+        .Select(x => double.TryParse(x, NumberStyles.Float, CultureInfo.InvariantCulture, out var value)
+            ? (double?)value : null)
+        .ToList();
+
+    private static List<GeoPoint> ParsePolygon(string raw) => raw
+        .Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries)
+        .Select(x => x.Split(','))
+        .Where(x => x.Length >= 2 &&
+                    double.TryParse(x[0], NumberStyles.Float, CultureInfo.InvariantCulture, out _) &&
+                    double.TryParse(x[1], NumberStyles.Float, CultureInfo.InvariantCulture, out _))
+        .Select(x => new GeoPoint(
+            double.Parse(x[0], CultureInfo.InvariantCulture),
+            double.Parse(x[1], CultureInfo.InvariantCulture)))
+        .ToList();
+
+    public static bool PointInPolygon(IReadOnlyList<GeoPoint> polygon, double latitude, double longitude)
+    {
+        var inside = false;
+        for (int i = 0, j = polygon.Count - 1; i < polygon.Count; j = i++)
+        {
+            var a = polygon[i];
+            var b = polygon[j];
+            if ((a.Latitude > latitude) != (b.Latitude > latitude) &&
+                longitude < (b.Longitude - a.Longitude) * (latitude - a.Latitude) /
+                            (b.Latitude - a.Latitude) + a.Longitude)
+                inside = !inside;
+        }
+        return inside;
+    }
+
+    private static bool Contains(IEnumerable<IReadOnlyList<GeoPoint>> polygons,
+        double latitude, double longitude) => polygons.Any(p => PointInPolygon(p, latitude, longitude));
+
+    private static double DistanceKm(double lat1, double lon1, double lat2, double lon2)
+    {
+        const double earthRadiusKm = 6371.0;
+        static double Rad(double degrees) => degrees * Math.PI / 180.0;
+        var dLat = Rad(lat2 - lat1);
+        var dLon = Rad(lon2 - lon1);
+        var a = Math.Sin(dLat / 2) * Math.Sin(dLat / 2) +
+                Math.Cos(Rad(lat1)) * Math.Cos(Rad(lat2)) *
+                Math.Sin(dLon / 2) * Math.Sin(dLon / 2);
+        return earthRadiusKm * 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));
+    }
+
+    private static DateTime? ParseDate(string? value) =>
+        DateTimeOffset.TryParse(value, CultureInfo.InvariantCulture, DateTimeStyles.AllowWhiteSpaces,
+            out var parsed) ? parsed.UtcDateTime : null;
+
+    private static double? Round(double? value, int digits) => value is null
+        ? null : Math.Round(value.Value, digits, MidpointRounding.AwayFromZero);
+
+    private static int SeverityRank(string severity) => severity switch
+    {
+        "Extreme" => 4, "Severe" => 3, "Moderate" => 2, "Minor" => 1, _ => 0,
+    };
+
+    public sealed record MosmixStation(string Id, string Name, double Latitude, double Longitude);
+    public sealed record ParsedForecast(DateTime? IssuedAt, List<WeatherForecastHour> Hours);
+    public sealed record GeoPoint(double Latitude, double Longitude);
+    public sealed record ParsedWarning(WeatherWarning Warning, List<List<GeoPoint>> Polygons);
+    private sealed record CachedForecast(DateTime LoadedAt, ParsedForecast Forecast);
+    private sealed record CachedWarnings(DateTime LoadedAt, List<ParsedWarning> Warnings);
+}

LehrerApp.Api/Endpoints/Endpoints.cs

@@ -1,9 +1,11 @@
 using System.IdentityModel.Tokens.Jwt;
 using System.Security.Claims;
 using System.Text;
+using System.Globalization;
 using LehrerApp.Core.Models;
 using LehrerApp.Sync.Models;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.RateLimiting;
 using Microsoft.IdentityModel.Tokens;
 
 namespace LehrerApp.Api;
@@ -14,21 +16,20 @@ public static class Endpoints
 
     public static void MapAuthEndpoints(this WebApplication app, string secret)
     {
-        app.MapPost("/api/auth/login", (LoginRequest req) =>
+        app.MapPost("/api/auth/login", (LoginRequest req, UserStore store) =>
         {
             if (string.IsNullOrWhiteSpace(req.Username) || string.IsNullOrWhiteSpace(req.Password))
                 return Results.Unauthorized();
-            // TODO: Passwort gegen DB prüfen
+            var user = store.Authenticate(req.Username, req.Password);
-            return Results.Ok(new { token = Jwt(req.Username, secret), userId = req.Username });
+            // Der kanonisch gespeicherte Username aus UserStore.Authenticate (nicht req.Username!)
-        });
+            // wird als JWT-userId verwendet - LiteDBs case-insensitive Standard-Collation lässt
-
+            // einen Login mit abweichender Groß-/Kleinschreibung erfolgreich durch; würde man
-        app.MapPost("/api/auth/register", (RegisterRequest req) =>
+            // stattdessen req.Username übernehmen, würde jede andersartig getippte Anmeldung einen
-        {
+            // eigenen, komplett getrennten Server-seitigen Event-Speicher erzeugen (siehe
-            if (req.Password.Length < 12)
+            // UserStore.Authenticate).
-                return Results.BadRequest("Passwort mind. 12 Zeichen.");
+            if (user is null) return Results.Unauthorized();
-            // TODO: User anlegen, Passwort hashen (BCrypt)
+            return Results.Ok(new { token = Jwt(user.Username, secret), userId = user.Username });
-            return Results.Ok(new { token = Jwt(req.Username, secret), userId = req.Username });
+        }).RequireRateLimiting("login");
-        });
     }
 
     // ── Sync ──────────────────────────────────────────────────────────────────
@@ -53,6 +54,41 @@ public static class Endpoints
             var uid = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
             return uid is null ? Results.Unauthorized() : Results.Ok(new { userId = uid, timestamp = DateTime.UtcNow });
         });
+        // Für Clients, deren Push wegen eines neueren Server-Stands abgelehnt wurde (10.3.4-
+        // Nachtrag: exakte Kollisionsprüfung statt 30s-Heuristik) — sofortiges Nachladen des
+        // aktuellen Stands EINER Entität, statt auf den nächsten regulären Pull zu warten.
+        g.MapGet("/entity/{entityType}/{entityId}", (string entityType, string entityId,
+            ClaimsPrincipal user, EventStore store) =>
+        {
+            var uid = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+            if (uid is null) return Results.Unauthorized();
+            var result = store.GetLatestForEntity(uid, entityType, entityId);
+            return result is null ? Results.NotFound() : Results.Ok(result);
+        });
+    }
+
+    // ── Anhänge (eigener Binärkanal, getrennt vom JSON-Ereigniskanal) ──────────
+
+    public static void MapAttachmentEndpoints(this WebApplication app)
+    {
+        var g = app.MapGroup("/api/sync/attachments").RequireAuthorization();
+        g.MapPost("/{storageId}", async (string storageId, HttpRequest req,
+            ClaimsPrincipal user, AttachmentStore store) =>
+        {
+            var uid = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+            if (uid is null) return Results.Unauthorized();
+            if (req.ContentLength is null or > LehrerApp.Core.Interfaces.IAttachmentStorage.MaxSizeBytes)
+                return Results.BadRequest("Datei zu groß oder Content-Length fehlt.");
+            await store.StoreAsync(uid, storageId, req.Body);
+            return Results.Ok();
+        });
+        g.MapGet("/{storageId}", (string storageId, ClaimsPrincipal user, AttachmentStore store) =>
+        {
+            var uid = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+            if (uid is null) return Results.Unauthorized();
+            var stream = store.OpenRead(uid, storageId);
+            return stream is null ? Results.NotFound() : Results.Stream(stream, "application/octet-stream");
+        });
     }
 
     // ── Snapshot (Device-Pairing) ─────────────────────────────────────────────
@@ -114,6 +150,123 @@ public static class Endpoints
         });
     }
 
+    // ── Schulstandort und DWD-Wetter ─────────────────────────────────────────
+
+    public static void MapSchoolWeatherEndpoints(this WebApplication app)
+    {
+        var g = app.MapGroup("/api/school").RequireAuthorization();
+
+        g.MapGet("/location", (ClaimsPrincipal user, SchoolWeatherStore store) =>
+        {
+            var uid = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+            if (uid is null) return Results.Unauthorized();
+            var location = store.GetLocation(uid);
+            return location is null
+                ? Results.NotFound("Noch kein Schulstandort hinterlegt.")
+                : Results.Ok(location);
+        });
+
+        g.MapPut("/location", async ([FromBody] SchoolLocationRequest request,
+            ClaimsPrincipal user, SchoolWeatherStore store, IGeocoder geocoder,
+            CancellationToken cancellationToken) =>
+        {
+            var uid = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+            if (uid is null) return Results.Unauthorized();
+
+            var errors = ValidateLocation(request);
+            if (errors.Count > 0) return Results.ValidationProblem(errors);
+
+            var existing = store.GetLocation(uid);
+            if (existing is not null && SameLocation(existing, request))
+                return Results.Ok(existing);
+
+            GeocodingResult? result;
+            try { result = await geocoder.GeocodeAsync(request, cancellationToken); }
+            catch (HttpRequestException)
+            {
+                return Results.Problem("Der Geocoding-Dienst ist momentan nicht erreichbar.",
+                    statusCode: StatusCodes.Status503ServiceUnavailable);
+            }
+            catch (TaskCanceledException) when (!cancellationToken.IsCancellationRequested)
+            {
+                return Results.Problem("Die Adressprüfung hat zu lange gedauert.",
+                    statusCode: StatusCodes.Status503ServiceUnavailable);
+            }
+            if (result is null)
+                return Results.UnprocessableEntity(new
+                {
+                    detail = "Die Schuladresse konnte nicht eindeutig gefunden werden. Bitte Schreibweise, PLZ und Ort prüfen.",
+                });
+
+            var profile = new SchoolLocationProfile
+            {
+                SchoolName = request.SchoolName.Trim(),
+                Street = request.Street.Trim(),
+                PostalCode = request.PostalCode.Trim(),
+                City = request.City.Trim(),
+                State = request.State,
+                Latitude = result.Latitude,
+                Longitude = result.Longitude,
+                ResolvedAddress = result.DisplayName,
+                UpdatedAt = DateTime.UtcNow,
+            };
+            store.SaveLocation(uid, profile);
+            return Results.Ok(profile);
+        });
+
+        g.MapGet("/weather", async (ClaimsPrincipal user, SchoolWeatherStore store,
+            DwdWeatherService weather, CancellationToken cancellationToken) =>
+        {
+            var uid = user.FindFirst(ClaimTypes.NameIdentifier)?.Value;
+            if (uid is null) return Results.Unauthorized();
+            var location = store.GetLocation(uid);
+            if (location is null) return Results.NotFound("Noch kein Schulstandort hinterlegt.");
+
+            var cached = store.GetWeather(uid);
+            if (cached is not null && DateTime.UtcNow - cached.RetrievedAt.ToUniversalTime() < TimeSpan.FromMinutes(5))
+                return Results.Ok(cached);
+            try
+            {
+                var current = await weather.GetAsync(location, cancellationToken);
+                store.SaveWeather(uid, current);
+                return Results.Ok(current);
+            }
+            catch (Exception ex) when (ex is HttpRequestException or InvalidDataException or InvalidOperationException)
+            {
+                if (cached is not null)
+                {
+                    cached.IsStale = true;
+                    return Results.Ok(cached);
+                }
+                return Results.Problem("Der DWD ist momentan nicht erreichbar und es liegt noch kein Wetter-Cache vor.",
+                    statusCode: StatusCodes.Status503ServiceUnavailable);
+            }
+        });
+    }
+
+    private static Dictionary<string, string[]> ValidateLocation(SchoolLocationRequest request)
+    {
+        var errors = new Dictionary<string, string[]>();
+        if (string.IsNullOrWhiteSpace(request.SchoolName))
+            errors[nameof(request.SchoolName)] = ["Bitte den Namen der Schule angeben."];
+        if (string.IsNullOrWhiteSpace(request.Street))
+            errors[nameof(request.Street)] = ["Bitte Straße und Hausnummer angeben."];
+        if (!System.Text.RegularExpressions.Regex.IsMatch(request.PostalCode?.Trim() ?? "", @"^\d{5}$"))
+            errors[nameof(request.PostalCode)] = ["Bitte eine fünfstellige deutsche PLZ angeben."];
+        if (string.IsNullOrWhiteSpace(request.City))
+            errors[nameof(request.City)] = ["Bitte den Ort angeben."];
+        if (!Enum.IsDefined(request.State))
+            errors[nameof(request.State)] = ["Das Bundesland ist ungültig."];
+        return errors;
+    }
+
+    private static bool SameLocation(SchoolLocationProfile existing, SchoolLocationRequest request) =>
+        string.Equals(existing.SchoolName, request.SchoolName.Trim(), StringComparison.OrdinalIgnoreCase) &&
+        string.Equals(existing.Street, request.Street.Trim(), StringComparison.OrdinalIgnoreCase) &&
+        string.Equals(existing.PostalCode, request.PostalCode.Trim(), StringComparison.Ordinal) &&
+        string.Equals(existing.City, request.City.Trim(), StringComparison.OrdinalIgnoreCase) &&
+        existing.State == request.State;
+
     // ── JWT ───────────────────────────────────────────────────────────────────
 
     private static string Jwt(string userId, string secret)
@@ -130,4 +283,3 @@ public static class Endpoints
 }
 
 public record LoginRequest(string Username, string Password);
-public record RegisterRequest(string Username, string Password, string DisplayName);

LehrerApp.Api/EventStore.cs

@@ -1,4 +1,5 @@
 using LiteDB;
+using LehrerApp.Sync;
 using LehrerApp.Sync.Models;
 
 namespace LehrerApp.Api;
@@ -11,37 +12,80 @@ public class EventStore(string dataPath) : IDisposable
     private readonly Dictionary<string, LiteDatabase> _dbs = new();
     private readonly Lock _lock = new();
 
+    /// <summary>
+    /// Nimmt Ereignisse an, wenn ihre <see cref="SyncEvent.BasedOnServerSeq"/> exakt der aktuellen
+    /// ServerSeq der jeweiligen Entität entspricht (null == Entität hier noch nie gesehen, z.B.
+    /// Neuanlage) — echte optimistische Nebenläufigkeitskontrolle statt der früheren 30-Sekunden-
+    /// Heuristik ("hat ein anderes Gerät kürzlich dieselbe Entität angefasst"), die sowohl falsch-
+    /// positiv (zwei Geräte bearbeiten zufällig kurz hintereinander verschiedene Felder) als auch
+    /// falsch-negativ (echter Konflikt liegt außerhalb des 30s-Fensters) sein konnte.
+    /// Mehrere Ereignisse derselben Entität IM SELBEN Aufruf bauen bewusst aufeinander auf (das
+    /// zweite prüft gegen den vom ersten gerade neu vergebenen Stand) — der Client schickt ohnehin
+    /// nur noch das jüngste ausstehende Ereignis je Entität (siehe SyncEngine.PushAsync).
+    /// </summary>
     public PushResponse Push(string userId, List<SyncEvent> events)
     {
         var col     = GetCol(userId);
         var seq     = LastSeq(col);
         var rejects  = new List<Guid>();
+        var assigned = new Dictionary<Guid, long>();
         foreach (var e in events.OrderBy(e => e.Timestamp))
         {
-            var recent = col.FindOne(x =>
+            var currentSeq = LatestForEntity(col, e.EntityType, e.EntityId)?.ServerSeq;
-                x.EntityType == e.EntityType && x.EntityId == e.EntityId &&
+            if (currentSeq != e.BasedOnServerSeq) { rejects.Add(e.EventId); continue; }
-                x.DeviceId != e.DeviceId && x.Timestamp > e.Timestamp.AddSeconds(-30));
+            var newSeq = ++seq;
-            if (recent is not null) { rejects.Add(e.EventId); continue; }
             col.Insert(new ServerEvent { EventId = e.EventId, DeviceId = e.DeviceId,
                 DeviceType = e.DeviceType, Timestamp = e.Timestamp,
-                ClientSeq = e.SequenceNr, ServerSeq = ++seq,
+                ClientSeq = e.SequenceNr, ServerSeq = newSeq,
                 EntityType = e.EntityType, EntityId = e.EntityId,
                 Operation = e.Operation, Payload = e.Payload });
+            assigned[e.EventId] = newSeq;
         }
-        return new() { Success = true, ServerSequenceNr = seq, ConflictingEventIds = rejects };
+        return new() { Success = true, ServerSequenceNr = seq, ConflictingEventIds = rejects,
+                        AssignedServerSeqs = assigned };
     }
 
+    /// <summary>Aktuellstes Ereignis einer einzelnen Entität — für Clients, deren Push wegen eines
+    /// neueren Server-Stands abgelehnt wurde (siehe <see cref="Push"/>), um sofort den aktuellen
+    /// Stand nachzuladen, statt auf den nächsten regulären Pull zu warten.</summary>
+    public SyncEvent? GetLatestForEntity(string userId, string entityType, string entityId)
+    {
+        var col = GetCol(userId);
+        var e = LatestForEntity(col, entityType, entityId);
+        // SequenceNr trägt hier (wie bei Pull) die ServerSeq — dieselbe Konvention wie sonst im
+        // Sync-Protokoll: bei vom Server stammenden Ereignissen ist SequenceNr immer die ServerSeq.
+        return e is null ? null : new SyncEvent { EventId = e.EventId, DeviceId = e.DeviceId,
+            DeviceType = e.DeviceType, Timestamp = e.Timestamp, SequenceNr = e.ServerSeq,
+            EntityType = e.EntityType, EntityId = e.EntityId, Operation = e.Operation,
+            Payload = e.Payload };
+    }
+
+    private static ServerEvent? LatestForEntity(ILiteCollection<ServerEvent> col, string entityType, string entityId) =>
+        col.Find(x => x.EntityType == entityType && x.EntityId == entityId)
+            .OrderByDescending(x => x.ServerSeq).FirstOrDefault();
+
     public PullResponse Pull(string userId, long since, string requestingDeviceId)
     {
         var col    = GetCol(userId);
         var events = col.Find(e => e.ServerSeq > since && e.DeviceId != requestingDeviceId)
-            .OrderBy(e => e.ServerSeq).Take(500)
+            .OrderBy(e => e.ServerSeq).Take(SyncProtocol.PullBatchSize)
             .Select(e => new SyncEvent { EventId = e.EventId, DeviceId = e.DeviceId,
                 DeviceType = e.DeviceType, Timestamp = e.Timestamp,
                 SequenceNr = e.ServerSeq, EntityType = e.EntityType,
                 EntityId = e.EntityId, Operation = e.Operation, Payload = e.Payload })
             .ToList();
-        return new() { Events = events, ServerSequenceNr = LastSeq(col) };
+        // ServerSequenceNr MUSS die höchste ServerSeq unter den tatsächlich zurückgegebenen
+        // Ereignissen sein, NICHT der globale Höchststand (LastSeq(col)) — der schließt auch
+        // Ereignisse ANDERER Geräte ein, die z.B. gerade erst (nach dem obigen Find-Aufruf, aber
+        // vor dieser Zeile) eingetroffen sind, oder — der Bug, der hier tatsächlich beobachtet
+        // wurde — Ereignisse des anfragenden Geräts selbst, die oben bewusst per
+        // "DeviceId != requestingDeviceId" herausgefiltert wurden. SyncEngine.PullAsync übernimmt
+        // ServerSequenceNr 1:1 als neuen "since"-Cursor für den nächsten Pull; mit dem globalen
+        // Höchststand würde der Client seinen Cursor über Ereignisse hinweg vorrücken, die er nie
+        // erhalten hat, und sie dauerhaft verpassen — genau das vom Nutzer beobachtete Symptom
+        // (Push meldet Erfolg, Pull liefert 0 Ereignisse, obwohl welche ausstehen).
+        var newWatermark = events.Count > 0 ? events.Max(e => e.SequenceNr) : since;
+        return new() { Events = events, ServerSequenceNr = newWatermark };
     }
 
     private ILiteCollection<ServerEvent> GetCol(string userId)

LehrerApp.Api/NominatimGeocoder.cs

@@ -0,0 +1,78 @@
+using System.Globalization;
+using System.Net.Http.Json;
+using System.Text.Json.Serialization;
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Api;
+
+public interface IGeocoder
+{
+    Task<GeocodingResult?> GeocodeAsync(SchoolLocationRequest address, CancellationToken cancellationToken);
+}
+
+public sealed record GeocodingResult(double Latitude, double Longitude, string DisplayName);
+
+/// <summary>Einmalige, explizit ausgelöste Adressauflösung über den öffentlichen OSM-Nominatim-
+/// Dienst. Kein Autocomplete; die dauerhafte Ergebnisspeicherung übernimmt SchoolWeatherStore.</summary>
+public sealed class NominatimGeocoder(HttpClient http) : IGeocoder
+{
+    private static readonly SemaphoreSlim RequestGate = new(1, 1);
+    private static DateTime _lastRequestAt = DateTime.MinValue;
+
+    public async Task<GeocodingResult?> GeocodeAsync(SchoolLocationRequest address,
+        CancellationToken cancellationToken)
+    {
+        var structured = Query(new Dictionary<string, string>
+        {
+            ["street"] = address.Street,
+            ["postalcode"] = address.PostalCode,
+            ["city"] = address.City,
+        });
+        var result = await SearchRespectingRateLimitAsync(structured, cancellationToken);
+        if (result is not null) return result;
+
+        // Manche Schulen/Ortsteile sind bei OSM anders adressiert. Ein kontrollierter zweiter
+        // Versuch mit der gesamten Suchzeile ist robuster, bleibt aber weit weg vom 1-rps-Limit.
+        var freeForm = string.Join(", ", new[]
+        {
+            address.SchoolName, address.Street, $"{address.PostalCode} {address.City}", "Deutschland",
+        }.Where(x => !string.IsNullOrWhiteSpace(x)));
+        return await SearchRespectingRateLimitAsync(
+            Query(new Dictionary<string, string> { ["q"] = freeForm }), cancellationToken);
+    }
+
+    private async Task<GeocodingResult?> SearchRespectingRateLimitAsync(string query,
+        CancellationToken cancellationToken)
+    {
+        await RequestGate.WaitAsync(cancellationToken);
+        try
+        {
+            var wait = TimeSpan.FromSeconds(1) - (DateTime.UtcNow - _lastRequestAt);
+            if (wait > TimeSpan.Zero) await Task.Delay(wait, cancellationToken);
+            _lastRequestAt = DateTime.UtcNow;
+            var candidates = await http.GetFromJsonAsync<List<NominatimCandidate>>(
+                $"search?format=jsonv2&limit=1&countrycodes=de{query}", cancellationToken);
+            var candidate = candidates?.FirstOrDefault();
+            if (candidate is null ||
+                !double.TryParse(candidate.Lat, NumberStyles.Float, CultureInfo.InvariantCulture, out var latitude) ||
+                !double.TryParse(candidate.Lon, NumberStyles.Float, CultureInfo.InvariantCulture, out var longitude))
+                return null;
+            return new(latitude, longitude, candidate.DisplayName ?? "");
+        }
+        finally { RequestGate.Release(); }
+    }
+
+    private static string Query(IReadOnlyDictionary<string, string> values) => string.Concat(
+        values.Where(x => !string.IsNullOrWhiteSpace(x.Value))
+              .Select(x => $"&{Uri.EscapeDataString(x.Key)}={Uri.EscapeDataString(x.Value.Trim())}"));
+
+    private sealed class NominatimCandidate
+    {
+        [JsonPropertyName("lat")]
+        public string? Lat { get; set; }
+        [JsonPropertyName("lon")]
+        public string? Lon { get; set; }
+        [JsonPropertyName("display_name")]
+        public string? DisplayName { get; set; }
+    }
+}

LehrerApp.Api/PasswordHasher.cs

@@ -0,0 +1,27 @@
+using System.Security.Cryptography;
+
+namespace LehrerApp.Api;
+
+public static class PasswordHasher
+{
+    private const int Iterations = 100_000;
+    private const int SaltSize = 16;
+    private const int HashSize = 32;
+
+    public static string Hash(string password)
+    {
+        var salt = RandomNumberGenerator.GetBytes(SaltSize);
+        var hash = Rfc2898DeriveBytes.Pbkdf2(password, salt, Iterations, HashAlgorithmName.SHA256, HashSize);
+        return $"{Convert.ToBase64String(salt)}:{Convert.ToBase64String(hash)}";
+    }
+
+    public static bool Verify(string password, string stored)
+    {
+        var parts = stored.Split(':');
+        if (parts.Length != 2) return false;
+        var salt = Convert.FromBase64String(parts[0]);
+        var expected = Convert.FromBase64String(parts[1]);
+        var actual = Rfc2898DeriveBytes.Pbkdf2(password, salt, Iterations, HashAlgorithmName.SHA256, HashSize);
+        return CryptographicOperations.FixedTimeEquals(actual, expected);
+    }
+}

LehrerApp.Api/PlainEventStore.cs

@@ -4,8 +4,10 @@ namespace LehrerApp.Api;
 
 public class PlainEventStore(EventStore eventStore)
 {
-    private static readonly HashSet<string> Allowed =
+    // Grade/ExamResult bewusst NICHT erlaubt: PlainSyncEvent.Payload ist Klartext-JSON (siehe
-        ["Grade", "ExamResult", "WorkTask", "Lesson"];
+    // SyncModels.cs), beide Typen tragen aber eine StudentId plus Notenwert/-kommentar -
+    // personenbezogene Daten dürfen laut TODO.md 10.3.3 nicht unverschlüsselt übertragen werden.
+    private static readonly HashSet<string> Allowed = ["WorkTask", "Lesson"];
 
     public PlainPushResponse Push(string userId, List<PlainSyncEvent> events)
     {

LehrerApp.Api/Program.cs

@@ -1,6 +1,9 @@
 using System.Text;
+using System.Threading.RateLimiting;
 using LehrerApp.Api;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
+using Microsoft.AspNetCore.HttpOverrides;
+using Microsoft.AspNetCore.RateLimiting;
 using Microsoft.IdentityModel.Tokens;
 
 var builder = WebApplication.CreateBuilder(args);
@@ -9,10 +12,22 @@ builder.WebHost.UseKestrel(o =>
 {
     var port = builder.Configuration.GetValue<int>("Api:Port", 5000);
     o.ListenAnyIP(port);
+    // Anhänge sind clientseitig auf 10 MB begrenzt (IAttachmentStorage.MaxSizeBytes) - etwas
+    // Puffer für Verschlüsselungs-Overhead/JSON-Ereignisse, aber trotzdem eine harte Obergrenze.
+    o.Limits.MaxRequestBodySize = 15 * 1024 * 1024;
 });
 
+var data = builder.Configuration["Api:DataPath"] ?? "./data";
+
+if (args.Length > 0 && args[0] == "create-user")
+    return await Cli.RunCreateUserAsync(data, args);
+if (args.Length > 0 && args[0] == "set-password")
+    return await Cli.RunSetPasswordAsync(data, args);
+
 var secret = builder.Configuration["JWT_SECRET"]
     ?? throw new InvalidOperationException("JWT_SECRET nicht konfiguriert.");
+var outboundUserAgent = builder.Configuration["Geocoding:UserAgent"]
+    ?? "LehrerApp-Server/1.0 (+https://science-teaching.de)";
 
 builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
     .AddJwtBearer(o => o.TokenValidationParameters = new()
@@ -24,19 +39,78 @@ builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
     });
 builder.Services.AddAuthorization();
 
-var data = builder.Configuration["Api:DataPath"] ?? "./data";
+// Läuft hinter einem Reverse Proxy (Dokploy/Traefik) - ohne diese Weiterleitung sähe Kestrel für
+// JEDE Anfrage dieselbe interne Proxy-IP als Connection.RemoteIpAddress, wodurch das unten
+// definierte Pro-IP-Rate-Limit faktisch zu einem einzigen globalen Limit für die gesamte
+// Bereitstellung würde (Bug, siehe TODO.md 10.2.2 Nachtrag). KnownNetworks/KnownProxies bewusst
+// geleert: der Container ist nur über den Reverse Proxy erreichbar, nie direkt aus dem Internet,
+// daher ist der unmittelbare Absender von X-Forwarded-For hier immer vertrauenswürdig.
+builder.Services.Configure<ForwardedHeadersOptions>(options =>
+{
+    options.ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto;
+    options.KnownIPNetworks.Clear();
+    options.KnownProxies.Clear();
+});
+
+// Rate Limiting (10.2.2): striktes Limit gezielt gegen Brute-Force auf /api/auth/login, plus ein
+// grobes globales Limit pro IP als einfacher Schutz vor Überlastung der übrigen Endpunkte.
+builder.Services.AddRateLimiter(options =>
+{
+    options.RejectionStatusCode = StatusCodes.Status429TooManyRequests;
+
+    options.AddFixedWindowLimiter("login", o =>
+    {
+        o.PermitLimit = 5;
+        o.Window = TimeSpan.FromMinutes(1);
+        o.QueueLimit = 0;
+    });
+
+    options.GlobalLimiter = PartitionedRateLimiter.Create<HttpContext, string>(context =>
+        RateLimitPartition.GetFixedWindowLimiter(
+            partitionKey: context.Connection.RemoteIpAddress?.ToString() ?? "unknown",
+            factory: _ => new FixedWindowRateLimiterOptions
+            {
+                PermitLimit = 120,
+                Window = TimeSpan.FromMinutes(1),
+                QueueLimit = 0,
+            }));
+});
+
+builder.Services.AddSingleton<UserStore>(_ => new UserStore(data));
+builder.Services.AddSingleton<AttachmentStore>(_ => new AttachmentStore(data));
 builder.Services.AddSingleton<EventStore>(_ => new EventStore(data));
 builder.Services.AddSingleton<SnapshotStore>(_ => new SnapshotStore(data));
 builder.Services.AddSingleton<ReadableSnapshotStore>(_ => new ReadableSnapshotStore(data));
 builder.Services.AddSingleton<PlainEventStore>(sp =>
     new PlainEventStore(sp.GetRequiredService<EventStore>()));
+builder.Services.AddSingleton<SchoolWeatherStore>(_ => new SchoolWeatherStore(data));
+builder.Services.AddHttpClient<IGeocoder, NominatimGeocoder>(client =>
+{
+    client.BaseAddress = new Uri("https://nominatim.openstreetmap.org/");
+    client.DefaultRequestHeaders.UserAgent.ParseAdd(outboundUserAgent);
+    client.Timeout = TimeSpan.FromSeconds(15);
+});
+builder.Services.AddHttpClient("dwd", client =>
+{
+    client.DefaultRequestHeaders.UserAgent.ParseAdd(outboundUserAgent);
+    client.Timeout = TimeSpan.FromSeconds(30);
+});
+builder.Services.AddSingleton(sp => new DwdWeatherService(
+    sp.GetRequiredService<IHttpClientFactory>().CreateClient("dwd")));
 
 var app = builder.Build();
+app.UseForwardedHeaders();
+app.UseRateLimiter();
+// Muss vor Auth/Endpoints laufen: inkompatible Clients dürfen keinen Sync-Store erreichen.
+app.UseMiddleware<SyncProtocolVersionMiddleware>();
 app.UseAuthentication();
 app.UseAuthorization();
 app.MapAuthEndpoints(secret);
 app.MapSyncEndpoints();
+app.MapAttachmentEndpoints();
 app.MapSnapshotEndpoints();
 app.MapReadableSnapshotEndpoints();
 app.MapPlainSyncEndpoints();
+app.MapSchoolWeatherEndpoints();
 app.Run();
+return 0;

LehrerApp.Api/SchoolWeatherStore.cs

@@ -0,0 +1,83 @@
+using LehrerApp.Core.Models;
+using LiteDB;
+
+namespace LehrerApp.Api;
+
+/// <summary>Unverschlüsselte Serverdaten, die der Server zur Abfrage standortbezogener öffentlicher
+/// Wetterdaten benötigt. Bewusst getrennt vom clientseitig verschlüsselten EventStore.</summary>
+public sealed class SchoolWeatherStore : IDisposable
+{
+    private readonly LiteDatabase _db;
+
+    public SchoolWeatherStore(string dataPath)
+    {
+        Directory.CreateDirectory(dataPath);
+        _db = new LiteDatabase(Path.Combine(dataPath, "school-weather.db"));
+    }
+
+    private ILiteCollection<LocationEntry> Locations
+    {
+        get
+        {
+            var col = _db.GetCollection<LocationEntry>("locations");
+            col.EnsureIndex(x => x.UserId, unique: true);
+            return col;
+        }
+    }
+
+    private ILiteCollection<WeatherEntry> Weather
+    {
+        get
+        {
+            var col = _db.GetCollection<WeatherEntry>("weather");
+            col.EnsureIndex(x => x.UserId, unique: true);
+            return col;
+        }
+    }
+
+    public SchoolLocationProfile? GetLocation(string userId) =>
+        Locations.FindOne(x => x.UserId == userId)?.Profile;
+
+    public void SaveLocation(string userId, SchoolLocationProfile profile)
+    {
+        var existing = Locations.FindOne(x => x.UserId == userId);
+        Locations.Upsert(new LocationEntry
+        {
+            Id = existing?.Id ?? ObjectId.NewObjectId(),
+            UserId = userId,
+            Profile = profile,
+        });
+        // Ein geänderter Standort darf niemals den Wetterstand des alten Orts liefern.
+        Weather.DeleteMany(x => x.UserId == userId);
+    }
+
+    public WeatherSnapshot? GetWeather(string userId) =>
+        Weather.FindOne(x => x.UserId == userId)?.Snapshot;
+
+    public void SaveWeather(string userId, WeatherSnapshot snapshot)
+    {
+        var existing = Weather.FindOne(x => x.UserId == userId);
+        Weather.Upsert(new WeatherEntry
+        {
+            Id = existing?.Id ?? ObjectId.NewObjectId(),
+            UserId = userId,
+            Snapshot = snapshot,
+        });
+    }
+
+    public void Dispose() => _db.Dispose();
+
+    private sealed class LocationEntry
+    {
+        public ObjectId Id { get; set; } = ObjectId.NewObjectId();
+        public string UserId { get; set; } = "";
+        public SchoolLocationProfile Profile { get; set; } = new();
+    }
+
+    private sealed class WeatherEntry
+    {
+        public ObjectId Id { get; set; } = ObjectId.NewObjectId();
+        public string UserId { get; set; } = "";
+        public WeatherSnapshot Snapshot { get; set; } = new();
+    }
+}

LehrerApp.Api/SnapshotStore.cs

@@ -19,6 +19,22 @@ public class SnapshotStore(string dataPath) : IDisposable
 
     public SnapshotUploadResponse Store(string userId, SnapshotUploadRequest req)
     {
+        // Zweiter Upload-Schritt (Schlüssel nachreichen, mit dem im ersten Schritt vergebenen Code
+        // verschlüsselt): denselben Eintrag aktualisieren statt einen neuen mit neuem Code
+        // anzulegen — sonst würde der Schlüssel dauerhaft mit dem falschen Code verknüpft bleiben
+        // und die Entschlüsselung auf dem Empfängergerät schlägt fehl (Bug, siehe TODO.md 10.3.1).
+        if (!string.IsNullOrEmpty(req.Code))
+        {
+            var existing = Col.FindOne(e => e.UserId == userId && e.Code == req.Code.ToUpperInvariant());
+            if (existing is not null)
+            {
+                existing.EncryptedPayload = req.EncryptedPayload;
+                existing.EncryptedSyncKey = req.EncryptedSyncKey;
+                Col.Update(existing);
+                return new() { Code = existing.Code, ExpiresAt = existing.ExpiresAt };
+            }
+        }
+
         Col.DeleteMany(e => e.UserId == userId);
         var code  = NewCode();
         var entry = new SnapshotEntry { Code = code, UserId = userId,

LehrerApp.Api/SyncProtocolVersionMiddleware.cs

@@ -0,0 +1,44 @@
+using System.Text.Json;
+using LehrerApp.Sync;
+
+namespace LehrerApp.Api;
+
+/// <summary>
+/// Stoppt veraltete Sync-Clients vor Authentifizierung und insbesondere vor jedem Store-Zugriff.
+/// Die Prüfung gilt auch für Anhänge, Plain-Sync und Device-Pairing-Snapshots.
+/// </summary>
+public sealed class SyncProtocolVersionMiddleware(RequestDelegate next)
+{
+    public async Task InvokeAsync(HttpContext context)
+    {
+        if (!IsProtectedSyncPath(context.Request.Path))
+        {
+            await next(context);
+            return;
+        }
+
+        context.Response.Headers[SyncProtocol.VersionHeaderName] = SyncProtocol.CurrentVersion;
+        var clientVersion = context.Request.Headers[SyncProtocol.VersionHeaderName].ToString();
+        if (string.Equals(clientVersion, SyncProtocol.CurrentVersion, StringComparison.Ordinal))
+        {
+            await next(context);
+            return;
+        }
+
+        context.Response.StatusCode = StatusCodes.Status426UpgradeRequired;
+        context.Response.ContentType = "application/problem+json";
+        await JsonSerializer.SerializeAsync(context.Response.Body, new
+        {
+            type = "https://lehrerapp.local/problems/sync-version-mismatch",
+            title = "Inkompatible Sync-Version",
+            status = StatusCodes.Status426UpgradeRequired,
+            detail = $"Der Server erwartet Sync-Version {SyncProtocol.CurrentVersion}.",
+            expectedVersion = SyncProtocol.CurrentVersion,
+            receivedVersion = string.IsNullOrEmpty(clientVersion) ? null : clientVersion,
+        });
+    }
+
+    private static bool IsProtectedSyncPath(PathString path) =>
+        path.StartsWithSegments("/api/sync", StringComparison.OrdinalIgnoreCase) ||
+        path.StartsWithSegments("/api/snapshot", StringComparison.OrdinalIgnoreCase);
+}

LehrerApp.Api/UserStore.cs

@@ -0,0 +1,61 @@
+using LiteDB;
+
+namespace LehrerApp.Api;
+
+public class UserStore(string dataPath) : IDisposable
+{
+    private readonly LiteDatabase _db = new(Path.Combine(dataPath, "users.db"));
+
+    private ILiteCollection<UserEntry> Col
+    {
+        get
+        {
+            var col = _db.GetCollection<UserEntry>("users");
+            col.EnsureIndex(x => x.Username, unique: true);
+            return col;
+        }
+    }
+
+    public bool CreateUser(string username, string password)
+    {
+        if (Col.Exists(x => x.Username == username)) return false;
+        Col.Insert(new UserEntry { Username = username, PasswordHash = PasswordHasher.Hash(password) });
+        return true;
+    }
+
+    public bool SetPassword(string username, string password)
+    {
+        var user = Col.FindOne(x => x.Username == username);
+        if (user is null) return false;
+        user.PasswordHash = PasswordHasher.Hash(password);
+        Col.Update(user);
+        return true;
+    }
+
+    /// <summary>
+    /// Prüft die Anmeldedaten und liefert bei Erfolg den GESPEICHERTEN Nutzereintrag zurück -
+    /// nicht bloß true/false. Wichtig: LiteDBs Standard-Collation vergleicht Strings (und damit
+    /// auch den eindeutigen Index auf Username) standardmäßig case-insensitive, d.h. ein Login mit
+    /// abweichender Groß-/Kleinschreibung authentifiziert erfolgreich. Würde der Aufrufer daraufhin
+    /// den roh eingegebenen Benutzernamen als userId für den Server-seitigen Event-Speicher
+    /// verwenden (siehe EventStore.GetCol), würde ein einziger andersartig getippter Login auf
+    /// einem zweiten Gerät einen komplett getrennten, nicht überlappenden Datenbestand erzeugen,
+    /// obwohl es sich aus Nutzersicht um dasselbe Konto handelt. Der kanonisch gespeicherte Name
+    /// aus diesem Rückgabewert stellt sicher, dass die userId unabhängig von der beim Login
+    /// eingegebenen Schreibweise stets identisch ist.
+    /// </summary>
+    public UserEntry? Authenticate(string username, string password)
+    {
+        var user = Col.FindOne(x => x.Username == username);
+        return user is not null && PasswordHasher.Verify(password, user.PasswordHash) ? user : null;
+    }
+
+    public void Dispose() => _db.Dispose();
+}
+
+public class UserEntry
+{
+    public ObjectId Id { get; set; } = ObjectId.NewObjectId();
+    public string Username { get; set; } = "";
+    public string PasswordHash { get; set; } = "";
+}

LehrerApp.Core/AiPlanning/AiPlanningDtos.cs

@@ -119,3 +119,58 @@ public class AiExplainResponse
 {
     public string Explanation { get; set; } = "";
 }
+
+/// <summary>
+/// Wire-Vertrag für den Gefährdungsbeurteilungs-Entwurf (ai-backend/gbu.php, Nutzerwunsch neben
+/// 4.2 "Anhänge je Stunde"): erkennt aus Thema/Verlaufsplan der Lesson das Experiment und liefert
+/// dafür einen strukturierten Entwurf. Eigener Endpunkt statt Zusatzfeld in plan.php/explain.php,
+/// da inhaltlich unabhängig von der Unterrichtsplanung selbst.
+/// </summary>
+public class AiHazardAssessmentRequest
+{
+    public AiUnitContext Unit { get; set; } = new();
+    public AiLesson Lesson { get; set; } = new();
+}
+
+public class AiHazardAssessmentResponse
+{
+    /// "Lehrerversuch" | "Schuelerversuch" | "Demonstrationsversuch" (siehe ExperimentKind).
+    public string ExperimentKind { get; set; } = "";
+    public string Procedure { get; set; } = "";
+    public List<AiHazardSubstance> Substances { get; set; } = [];
+    public List<string> Hazards { get; set; } = [];
+    public List<string> ProtectiveMeasures { get; set; } = [];
+    public string FirstAid { get; set; } = "";
+    public string Disposal { get; set; } = "";
+}
+
+public class AiHazardSubstance
+{
+    public string Name { get; set; } = "";
+    public string Amount { get; set; } = "";
+    /// GHS-Piktogramm-Codes als Text, z.B. "GHS05" — siehe GhsPictogram für die Zuordnung.
+    public List<string> GhsPictograms { get; set; } = [];
+    public string HStatements { get; set; } = "";
+    public string PStatements { get; set; } = "";
+}
+
+/// <summary>
+/// Wire-Vertrag für die Chemikalien-Recherche (ai-backend/substance.php, Nutzerwunsch neben 4.2):
+/// ein reiner Datenbank-Lookup gegen die lokale RiSU-Stoffliste, kein KI-Aufruf (siehe
+/// ai-backend/stoffliste.php) — der Name "Ai..." bleibt trotzdem bestehen, da der Aufruf über
+/// dieselbe Anmeldung/denselben Host wie die echten KI-Endpunkte läuft. Eigener, sehr schmaler
+/// Wire-Vertrag statt Wiederverwendung von AiHazardSubstance — die Recherche liefert
+/// zusätzliche, nur hier relevante Felder (CAS, Signalwort, Tätigkeitsbeschränkung, Quelle).
+/// Enthält bewusst KEIN Entsorgungsfeld — die RiSU-Stoffliste führt keine Entsorgungshinweise.
+/// </summary>
+public class AiSubstanceResearchResponse
+{
+    public string Name { get; set; } = "";
+    public string Cas { get; set; } = "";
+    public List<string> GhsPictograms { get; set; } = [];
+    public string SignalWord { get; set; } = "";
+    public string HStatements { get; set; } = "";
+    public string PStatements { get; set; } = "";
+    public string ActivityRestriction { get; set; } = "";
+    public string Source { get; set; } = "";
+}

LehrerApp.Core/Importing/DelimitedTextParser.cs

@@ -0,0 +1,77 @@
+using System.Text;
+
+namespace LehrerApp.Core.Importing;
+
+internal sealed record DelimitedRecord(int LineNumber, IReadOnlyList<string> Fields);
+
+internal static class DelimitedTextParser
+{
+    public static bool TryParse(
+        string text,
+        char delimiter,
+        out List<DelimitedRecord> records,
+        out string? error)
+    {
+        records = [];
+        error = null;
+        var fields = new List<string>();
+        var field = new StringBuilder();
+        var inQuotes = false;
+        var line = 1;
+        var recordStartLine = 1;
+
+        for (var index = 0; index < text.Length; index++)
+        {
+            var current = text[index];
+            if (current == '"')
+            {
+                if (inQuotes && index + 1 < text.Length && text[index + 1] == '"')
+                {
+                    field.Append('"');
+                    index++;
+                }
+                else
+                {
+                    inQuotes = !inQuotes;
+                }
+                continue;
+            }
+
+            if (current == delimiter && !inQuotes)
+            {
+                fields.Add(field.ToString());
+                field.Clear();
+                continue;
+            }
+
+            if ((current == '\r' || current == '\n') && !inQuotes)
+            {
+                if (current == '\r' && index + 1 < text.Length && text[index + 1] == '\n')
+                    index++;
+                fields.Add(field.ToString());
+                field.Clear();
+                records.Add(new DelimitedRecord(recordStartLine, fields.ToList()));
+                fields.Clear();
+                line++;
+                recordStartLine = line;
+                continue;
+            }
+
+            if (current == '\n') line++;
+            field.Append(current);
+        }
+
+        if (inQuotes)
+        {
+            error = $"Ein Textfeld ab Zeile {recordStartLine} besitzt kein schließendes Anführungszeichen.";
+            return false;
+        }
+
+        if (field.Length > 0 || fields.Count > 0)
+        {
+            fields.Add(field.ToString());
+            records.Add(new DelimitedRecord(recordStartLine, fields.ToList()));
+        }
+        return true;
+    }
+}

LehrerApp.Core/Importing/ImportAbstractions.cs

@@ -0,0 +1,210 @@
+using System.Collections.ObjectModel;
+
+namespace LehrerApp.Core.Importing;
+
+/// <summary>
+/// Stabile, typsichere Kennung eines Importformats. Werte werden normalisiert, damit
+/// Groß-/Kleinschreibung oder versehentliche Leerzeichen nicht zu Tippfehlern führen.
+/// </summary>
+public readonly record struct ImportFormatId
+{
+    public string Value { get; }
+
+    public ImportFormatId(string value)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(value);
+        Value = value.Trim().ToLowerInvariant();
+    }
+
+    public override string ToString() => Value ?? "";
+}
+
+/// <summary>Eine vollständig in den Speicher geladene Importdatei.</summary>
+public sealed class ImportFile
+{
+    public string FileName { get; }
+    public ReadOnlyMemory<byte> Content { get; }
+    public string Extension => Path.GetExtension(FileName).ToLowerInvariant();
+
+    public ImportFile(string fileName, ReadOnlyMemory<byte> content)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(fileName);
+        FileName = Path.GetFileName(fileName);
+        Content = content;
+    }
+}
+
+public enum ImportMessageSeverity { Information, Warning, Error }
+
+public sealed record ImportMessage(
+    ImportMessageSeverity Severity,
+    string Code,
+    string Message,
+    string? SourceReference = null);
+
+/// <summary>
+/// Ergebnis einer schnellen Formaterkennung. 0 bedeutet „kein Treffer“, 100 einen
+/// eindeutigen Treffer. Die eigentliche vollständige Prüfung erfolgt erst beim Parsen.
+/// </summary>
+public sealed record ImportDetection
+{
+    public int Confidence { get; }
+    public string? Reason { get; }
+    public bool IsMatch => Confidence > 0;
+
+    public ImportDetection(int confidence, string? reason = null)
+    {
+        if (confidence is < 0 or > 100)
+            throw new ArgumentOutOfRangeException(nameof(confidence));
+        Confidence = confidence;
+        Reason = reason;
+    }
+
+    public static ImportDetection NoMatch(string? reason = null) => new(0, reason);
+    public static ImportDetection Match(int confidence, string? reason = null) =>
+        new(confidence, reason);
+}
+
+public sealed class ImportParseResult<T>
+{
+    public IReadOnlyList<T> Items { get; }
+    public IReadOnlyList<ImportMessage> Messages { get; }
+    public bool IsValid => Messages.All(message => message.Severity != ImportMessageSeverity.Error);
+
+    public ImportParseResult(IEnumerable<T> items, IEnumerable<ImportMessage>? messages = null)
+    {
+        ArgumentNullException.ThrowIfNull(items);
+        Items = ReadOnly(items);
+        Messages = ReadOnly(messages ?? []);
+    }
+
+    private static IReadOnlyList<TItem> ReadOnly<TItem>(IEnumerable<TItem> values) =>
+        new ReadOnlyCollection<TItem>(values.ToList());
+}
+
+/// <summary>
+/// Ein registrierbarer, UI-unabhängiger Handler für genau ein Dateiformat und ein
+/// normalisiertes fachliches Importmodell.
+/// </summary>
+public interface IImportHandler<T>
+{
+    ImportFormatId FormatId { get; }
+    string DisplayName { get; }
+    IReadOnlyCollection<string> SupportedExtensions { get; }
+
+    ValueTask<ImportDetection> DetectAsync(
+        ImportFile file,
+        CancellationToken cancellationToken = default);
+
+    ValueTask<ImportParseResult<T>> ParseAsync(
+        ImportFile file,
+        CancellationToken cancellationToken = default);
+}
+
+public sealed record DetectedImportHandler<T>(
+    IImportHandler<T> Handler,
+    ImportDetection Detection);
+
+public abstract class ImportFormatException(string message) : Exception(message);
+
+public sealed class UnsupportedImportFormatException(string fileName)
+    : ImportFormatException($"Für die Datei „{fileName}“ wurde kein passendes Importformat gefunden.");
+
+public sealed class AmbiguousImportFormatException(string fileName, IEnumerable<string> formats)
+    : ImportFormatException(
+        $"Die Datei „{fileName}“ passt zu mehreren Importformaten: {string.Join(", ", formats)}.");
+
+/// <summary>
+/// Verwaltet Handler eines fachlichen Importziels und wählt anhand des Dateiinhalts
+/// den besten Treffer. Die Dateiendung dient bei gleicher Erkennungsqualität als Hinweis.
+/// </summary>
+public sealed class ImportHandlerCatalog<T>
+{
+    private readonly IReadOnlyList<IImportHandler<T>> _handlers;
+
+    public ImportHandlerCatalog(IEnumerable<IImportHandler<T>> handlers)
+    {
+        ArgumentNullException.ThrowIfNull(handlers);
+        var materialized = handlers.ToList();
+        var duplicate = materialized
+            .GroupBy(handler => handler.FormatId)
+            .FirstOrDefault(group => group.Count() > 1);
+        if (duplicate is not null)
+            throw new ArgumentException(
+                $"Das Importformat „{duplicate.Key}“ wurde mehrfach registriert.", nameof(handlers));
+        _handlers = new ReadOnlyCollection<IImportHandler<T>>(materialized);
+    }
+
+    public IReadOnlyList<IImportHandler<T>> Handlers => _handlers;
+
+    public IReadOnlyList<string> SupportedExtensions => _handlers
+        .SelectMany(handler => handler.SupportedExtensions)
+        .Select(NormalizeExtension)
+        .Where(extension => extension.Length > 0)
+        .Distinct(StringComparer.OrdinalIgnoreCase)
+        .OrderBy(extension => extension, StringComparer.OrdinalIgnoreCase)
+        .ToList()
+        .AsReadOnly();
+
+    public async ValueTask<DetectedImportHandler<T>> DetectAsync(
+        ImportFile file,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(file);
+        var matches = new List<Candidate>();
+        foreach (var handler in _handlers)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            var detection = await handler.DetectAsync(file, cancellationToken).ConfigureAwait(false);
+            if (detection.IsMatch)
+                matches.Add(new Candidate(handler, detection, SupportsExtension(handler, file.Extension)));
+        }
+
+        if (matches.Count == 0)
+            throw new UnsupportedImportFormatException(file.FileName);
+
+        var highestConfidence = matches.Max(match => match.Detection.Confidence);
+        var best = matches.Where(match => match.Detection.Confidence == highestConfidence).ToList();
+        if (best.Count > 1 && best.Any(match => match.ExtensionMatches))
+            best = best.Where(match => match.ExtensionMatches).ToList();
+
+        if (best.Count != 1)
+            throw new AmbiguousImportFormatException(
+                file.FileName,
+                best.Select(match => match.Handler.DisplayName));
+
+        return new DetectedImportHandler<T>(best[0].Handler, best[0].Detection);
+    }
+
+    private static bool SupportsExtension(IImportHandler<T> handler, string extension) =>
+        handler.SupportedExtensions
+            .Select(NormalizeExtension)
+            .Contains(extension, StringComparer.OrdinalIgnoreCase);
+
+    private static string NormalizeExtension(string extension)
+    {
+        if (string.IsNullOrWhiteSpace(extension)) return "";
+        var trimmed = extension.Trim().ToLowerInvariant();
+        return trimmed.StartsWith('.') ? trimmed : $".{trimmed}";
+    }
+
+    private sealed record Candidate(
+        IImportHandler<T> Handler,
+        ImportDetection Detection,
+        bool ExtensionMatches);
+}
+
+public sealed record ImportConflictOption(string Id, string Label, string? Description = null);
+
+public sealed record ImportConflict(
+    string Id,
+    string Title,
+    string Description,
+    string ImportedValue,
+    string? ExistingValue,
+    IReadOnlyList<ImportConflictOption> Options,
+    string DefaultOptionId,
+    string? SourceReference = null);
+
+/// <summary>Vom UI getroffene, für den Core rein datenförmige Konfliktentscheidung.</summary>
+public sealed record ImportDecision(string ConflictId, string SelectedOptionId);

LehrerApp.Core/Importing/LessonStudentListCsvImportHandler.cs

@@ -0,0 +1,121 @@
+using System.Text;
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Importing;
+
+/// <summary>Importiert den tabulatorgetrennten Export „Teilnehmerliste“ eines Unterrichts.</summary>
+public sealed class LessonStudentListCsvImportHandler : IImportHandler<ImportedStudent>
+{
+    private static readonly string[] SignatureHeaders =
+        ["Langname", "Vorname", "Klasse", "Geschlecht", "Eintrittsdatum", "Austrittsdatum",
+         "E-Mail Adresse", "Mobiltelefon", "Telefonnummer"];
+    private static readonly UTF8Encoding StrictUtf8 = new(false, true);
+
+    public ImportFormatId FormatId => StudentImportFormats.LessonStudentListCsv;
+    public string DisplayName => "Unterrichts-Teilnehmerliste";
+    public IReadOnlyCollection<string> SupportedExtensions { get; } = [".csv"];
+
+    public ValueTask<ImportDetection> DetectAsync(ImportFile file, CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        if (!TryDecode(file, out var text))
+            return ValueTask.FromResult(ImportDetection.NoMatch("Die Datei ist kein gültiges UTF-8."));
+        var headers = FirstLine(text).Split('\t').Select(x => x.Trim())
+            .ToHashSet(StringComparer.OrdinalIgnoreCase);
+        return ValueTask.FromResult(SignatureHeaders.All(headers.Contains)
+            ? ImportDetection.Match(file.Extension.Equals(".csv", StringComparison.OrdinalIgnoreCase) ? 100 : 95,
+                "Die charakteristischen Spalten der Unterrichts-Teilnehmerliste wurden gefunden.")
+            : ImportDetection.NoMatch("Die erwarteten Spalten der Unterrichts-Teilnehmerliste fehlen."));
+    }
+
+    public ValueTask<ImportParseResult<ImportedStudent>> ParseAsync(ImportFile file, CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        var messages = new List<ImportMessage>();
+        if (!TryDecode(file, out var text))
+            return Result([], [Error("lesson-students.encoding", "Die Teilnehmerliste ist nicht als gültiges UTF-8 gespeichert.")]);
+        if (!DelimitedTextParser.TryParse(text, '\t', out var records, out var parseError))
+            return Result([], [Error("lesson-students.structure", parseError ?? "Die Teilnehmerliste ist syntaktisch ungültig.")]);
+        if (records.Count == 0)
+            return Result([], [Error("lesson-students.empty", "Die Teilnehmerliste ist leer.")]);
+
+        var header = records[0].Fields.Select((name, index) => (Name: name.Trim().TrimStart('\uFEFF'), Index: index)).ToList();
+        var duplicate = header.GroupBy(x => x.Name, StringComparer.OrdinalIgnoreCase).FirstOrDefault(x => x.Count() > 1);
+        if (duplicate is not null)
+            messages.Add(Error("lesson-students.header.duplicate", $"Die Spalte „{duplicate.Key}“ kommt mehrfach vor.", "Kopfzeile"));
+        var columns = header.GroupBy(x => x.Name, StringComparer.OrdinalIgnoreCase)
+            .ToDictionary(x => x.Key, x => x.First().Index, StringComparer.OrdinalIgnoreCase);
+        foreach (var required in SignatureHeaders.Where(x => !columns.ContainsKey(x)))
+            messages.Add(Error("lesson-students.header.missing", $"Die erforderliche Spalte „{required}“ fehlt.", "Kopfzeile"));
+        if (messages.Any(x => x.Severity == ImportMessageSeverity.Error)) return Result([], messages);
+
+        var students = new List<ImportedStudent>();
+        foreach (var record in records.Skip(1))
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            if (record.Fields.All(string.IsNullOrWhiteSpace)) continue;
+            var source = $"Zeile {record.LineNumber}";
+            if (record.Fields.Count != header.Count)
+            {
+                messages.Add(Error("lesson-students.columns",
+                    $"Die Zeile enthält {record.Fields.Count} statt {header.Count} Spalten.", source));
+                continue;
+            }
+            students.Add(new ImportedStudent
+            {
+                LastName = Value(record, columns, "Langname"),
+                FirstName = Value(record, columns, "Vorname"),
+                GroupName = NullIfEmpty(Value(record, columns, "Klasse")),
+                Gender = ParseGender(Value(record, columns, "Geschlecht"), source, messages),
+                Contact = new ImportedStudentContact
+                {
+                    Email = NullIfEmpty(Value(record, columns, "E-Mail Adresse")),
+                    MobilePhone = NullIfEmpty(Value(record, columns, "Mobiltelefon")),
+                    Phone = NullIfEmpty(Value(record, columns, "Telefonnummer")),
+                },
+                SourceReference = source,
+            });
+        }
+        return Result(students, messages);
+    }
+
+    private static Gender? ParseGender(string value, string source, ICollection<ImportMessage> messages)
+    {
+        if (string.IsNullOrWhiteSpace(value)) return null;
+        return value.Trim().ToLowerInvariant() switch
+        {
+            "m" or "männlich" or "maennlich" => Gender.M,
+            "w" or "weiblich" => Gender.W,
+            "d" or "divers" => Gender.D,
+            _ => UnknownGender(value, source, messages),
+        };
+    }
+
+    private static Gender? UnknownGender(string value, string source, ICollection<ImportMessage> messages)
+    {
+        messages.Add(new ImportMessage(ImportMessageSeverity.Warning, "lesson-students.gender.unknown",
+            $"Das Geschlecht „{value.Trim()}“ ist unbekannt und wird nicht übernommen.", source));
+        return null;
+    }
+
+    private static bool TryDecode(ImportFile file, out string text)
+    {
+        try { text = StrictUtf8.GetString(file.Content.Span); return true; }
+        catch (DecoderFallbackException) { text = ""; return false; }
+    }
+
+    private static string FirstLine(string text)
+    {
+        var end = text.IndexOfAny(['\r', '\n']);
+        return (end >= 0 ? text[..end] : text).TrimStart('\uFEFF');
+    }
+
+    private static string Value(DelimitedRecord row, IReadOnlyDictionary<string, int> columns, string name) =>
+        columns.TryGetValue(name, out var index) && index < row.Fields.Count ? row.Fields[index].Trim() : "";
+    private static string? NullIfEmpty(string value) => string.IsNullOrWhiteSpace(value) ? null : value.Trim();
+    private static ImportMessage Error(string code, string message, string? source = null) =>
+        new(ImportMessageSeverity.Error, code, message, source);
+    private static ValueTask<ImportParseResult<ImportedStudent>> Result(
+        IReadOnlyList<ImportedStudent> students, IReadOnlyList<ImportMessage> messages) =>
+        ValueTask.FromResult(new ImportParseResult<ImportedStudent>(students, messages));
+}

LehrerApp.Core/Importing/MarksPerLessonCsvImportHandler.cs

@@ -0,0 +1,40 @@
+using System.Text;
+
+namespace LehrerApp.Core.Importing;
+
+/// <summary>Erkennt den Notenlistenexport und erklärt dessen fehlende Teilnehmerdaten.</summary>
+public sealed class MarksPerLessonCsvImportHandler : IImportHandler<ImportedStudent>
+{
+    private static readonly string[] SignatureHeaders =
+        ["Datum", "Name", "Klasse", "Fach", "Prüfungsart", "Note", "Bemerkung", "Benutzer", "Schlüssel (extern)", "Gesamtnote"];
+    private static readonly UTF8Encoding StrictUtf8 = new(false, true);
+
+    public ImportFormatId FormatId => StudentImportFormats.MarksPerLessonCsv;
+    public string DisplayName => "Unterrichts-Notenliste";
+    public IReadOnlyCollection<string> SupportedExtensions { get; } = [".csv"];
+
+    public ValueTask<ImportDetection> DetectAsync(ImportFile file, CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        string text;
+        try { text = StrictUtf8.GetString(file.Content.Span); }
+        catch (DecoderFallbackException) { return ValueTask.FromResult(ImportDetection.NoMatch()); }
+        var end = text.IndexOfAny(['\r', '\n']);
+        var firstLine = (end >= 0 ? text[..end] : text).TrimStart('\uFEFF');
+        var headers = firstLine.Split('\t').Select(x => x.Trim()).ToHashSet(StringComparer.OrdinalIgnoreCase);
+        return ValueTask.FromResult(SignatureHeaders.All(headers.Contains)
+            ? ImportDetection.Match(file.Extension.Equals(".csv", StringComparison.OrdinalIgnoreCase) ? 100 : 95)
+            : ImportDetection.NoMatch());
+    }
+
+    public ValueTask<ImportParseResult<ImportedStudent>> ParseAsync(ImportFile file, CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        IReadOnlyList<ImportMessage> messages =
+        [
+            new(ImportMessageSeverity.Error, "marks-per-lesson.no-participants",
+                "Die Notenliste enthält keine verlässlichen Teilnehmerdaten. Bitte exportiere die echte Kurs-/Teilnehmerliste.")
+        ];
+        return ValueTask.FromResult(new ImportParseResult<ImportedStudent>([], messages));
+    }
+}

LehrerApp.Core/Importing/StudentImportModels.cs

@@ -0,0 +1,104 @@
+using System.Collections.ObjectModel;
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Importing;
+
+public sealed record ImportedStudentContact
+{
+    public string? Email { get; init; }
+    public string? MobilePhone { get; init; }
+    public string? Phone { get; init; }
+    public string? Street { get; init; }
+    public string? PostalCode { get; init; }
+    public string? City { get; init; }
+
+    public bool HasData => new[] { Email, MobilePhone, Phone, Street, PostalCode, City }
+        .Any(value => !string.IsNullOrWhiteSpace(value));
+}
+
+/// <summary>Formatunabhängige Schülerdaten, die ein konkreter Handler erzeugt.</summary>
+public sealed record ImportedStudent
+{
+    public required string FirstName { get; init; }
+    public required string LastName { get; init; }
+    public DateOnly? DateOfBirth { get; init; }
+    public Gender? Gender { get; init; }
+    public string? ExternalId { get; init; }
+    public string? GroupName { get; init; }
+    public ImportedStudentContact? Contact { get; init; }
+    public string? SourceReference { get; init; }
+}
+
+public static class StudentImportFormats
+{
+    public static readonly ImportFormatId MasterDataCsv = new("student-master-data.csv");
+    public static readonly ImportFormatId LessonStudentListCsv = new("lesson-student-list.csv");
+    public static readonly ImportFormatId MarksPerLessonCsv = new("marks-per-lesson.csv");
+}
+
+public enum StudentImportResolutionKind { CreateNew, UseExisting, Skip }
+
+public sealed record StudentImportResolution(
+    StudentImportResolutionKind Kind,
+    Guid? ExistingStudentId = null);
+
+public sealed record StudentImportEntry(
+    string Id,
+    ImportedStudent Student,
+    StudentImportResolution? AutomaticResolution,
+    string? ConflictId,
+    IReadOnlyList<string> FieldsToSupplement);
+
+public sealed record StudentImportGroupAssignment(
+    string SourceGroupName,
+    Guid? AutomaticGroupId,
+    string? ConflictId);
+
+public sealed class StudentImportPreview
+{
+    public ImportFormatId FormatId { get; }
+    public string FormatName { get; }
+    public IReadOnlyList<StudentImportEntry> Entries { get; }
+    public IReadOnlyList<ImportMessage> Messages { get; }
+    public IReadOnlyList<ImportConflict> Conflicts { get; }
+    public IReadOnlyList<StudentImportGroupAssignment> GroupAssignments { get; }
+    public Guid? TargetGroupId { get; }
+    internal string ExistingStudentsFingerprint { get; }
+    internal string GroupStateFingerprint { get; }
+
+    public bool CanApply =>
+        Entries.Count > 0 && Messages.All(message => message.Severity != ImportMessageSeverity.Error);
+
+    internal StudentImportPreview(
+        ImportFormatId formatId,
+        string formatName,
+        IEnumerable<StudentImportEntry> entries,
+        IEnumerable<ImportMessage> messages,
+        IEnumerable<ImportConflict> conflicts,
+        IEnumerable<StudentImportGroupAssignment> groupAssignments,
+        Guid? targetGroupId,
+        string existingStudentsFingerprint,
+        string groupStateFingerprint)
+    {
+        FormatId = formatId;
+        FormatName = formatName;
+        Entries = ReadOnly(entries);
+        Messages = ReadOnly(messages);
+        Conflicts = ReadOnly(conflicts);
+        GroupAssignments = ReadOnly(groupAssignments);
+        TargetGroupId = targetGroupId;
+        ExistingStudentsFingerprint = existingStudentsFingerprint;
+        GroupStateFingerprint = groupStateFingerprint;
+    }
+
+    private static IReadOnlyList<T> ReadOnly<T>(IEnumerable<T> values) =>
+        new ReadOnlyCollection<T>(values.ToList());
+}
+
+public sealed record StudentImportApplyResult(
+    int CreatedStudents,
+    int UpdatedStudents,
+    int MatchedExistingStudents,
+    int SkippedStudents,
+    int CreatedMemberships,
+    IReadOnlyList<Guid> CreatedStudentIds);

LehrerApp.Core/Importing/StudentMasterDataCsvImportHandler.cs

@@ -0,0 +1,225 @@
+using System.Globalization;
+using System.Text;
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Importing;
+
+/// <summary>
+/// Liest den tabulatorgetrennten Stammdatenexport mit Spalten wie „longName“,
+/// „foreName“, „birthDate“ und „klasse.name“. Die Datei trägt historisch die
+/// Endung .csv, obwohl als Trennzeichen ein Tabulator verwendet wird.
+/// </summary>
+public sealed class StudentMasterDataCsvImportHandler : IImportHandler<ImportedStudent>
+{
+    private static readonly string[] SignatureHeaders =
+        ["longName", "foreName", "gender", "birthDate", "klasse.name", "externKey"];
+
+    private static readonly UTF8Encoding StrictUtf8 = new(
+        encoderShouldEmitUTF8Identifier: false,
+        throwOnInvalidBytes: true);
+
+    public ImportFormatId FormatId => StudentImportFormats.MasterDataCsv;
+    public string DisplayName => "Schüler-Stammdatenliste";
+    public IReadOnlyCollection<string> SupportedExtensions { get; } = [".csv"];
+
+    public ValueTask<ImportDetection> DetectAsync(
+        ImportFile file,
+        CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        string text;
+        try
+        {
+            text = StrictUtf8.GetString(file.Content.Span);
+        }
+        catch (DecoderFallbackException)
+        {
+            return ValueTask.FromResult(ImportDetection.NoMatch("Die Datei ist kein gültiges UTF-8."));
+        }
+
+        var firstLineEnd = text.IndexOfAny(['\r', '\n']);
+        var firstLine = (firstLineEnd >= 0 ? text[..firstLineEnd] : text).TrimStart('\uFEFF');
+        var headers = firstLine.Split('\t')
+            .Select(header => header.Trim())
+            .ToHashSet(StringComparer.OrdinalIgnoreCase);
+        if (!SignatureHeaders.All(headers.Contains))
+            return ValueTask.FromResult(ImportDetection.NoMatch("Die erwarteten Stammdatenspalten fehlen."));
+
+        return ValueTask.FromResult(ImportDetection.Match(
+            file.Extension.Equals(".csv", StringComparison.OrdinalIgnoreCase) ? 100 : 95,
+            "Die charakteristischen Stammdatenspalten wurden gefunden."));
+    }
+
+    public ValueTask<ImportParseResult<ImportedStudent>> ParseAsync(
+        ImportFile file,
+        CancellationToken cancellationToken = default)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        var messages = new List<ImportMessage>();
+        string text;
+        try
+        {
+            text = StrictUtf8.GetString(file.Content.Span);
+        }
+        catch (DecoderFallbackException)
+        {
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "master-data.encoding",
+                "Die Stammdatenliste ist nicht als gültiges UTF-8 gespeichert."));
+            return ValueTask.FromResult(new ImportParseResult<ImportedStudent>([], messages));
+        }
+
+        if (!DelimitedTextParser.TryParse(text, '\t', out var records, out var parseError))
+        {
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "master-data.structure",
+                parseError ?? "Die Stammdatenliste ist syntaktisch ungültig."));
+            return ValueTask.FromResult(new ImportParseResult<ImportedStudent>([], messages));
+        }
+
+        if (records.Count == 0)
+        {
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "master-data.empty",
+                "Die Stammdatenliste ist leer."));
+            return ValueTask.FromResult(new ImportParseResult<ImportedStudent>([], messages));
+        }
+
+        var headers = records[0].Fields
+            .Select((header, index) => (Name: header.Trim().TrimStart('\uFEFF'), Index: index))
+            .ToList();
+        var duplicateHeader = headers
+            .GroupBy(header => header.Name, StringComparer.OrdinalIgnoreCase)
+            .FirstOrDefault(group => group.Count() > 1);
+        if (duplicateHeader is not null)
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "master-data.header.duplicate",
+                $"Die Spalte „{duplicateHeader.Key}“ kommt mehrfach vor.",
+                "Kopfzeile"));
+
+        var columns = headers
+            .GroupBy(header => header.Name, StringComparer.OrdinalIgnoreCase)
+            .ToDictionary(group => group.Key, group => group.First().Index, StringComparer.OrdinalIgnoreCase);
+        foreach (var required in SignatureHeaders.Where(required => !columns.ContainsKey(required)))
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "master-data.header.missing",
+                $"Die erforderliche Spalte „{required}“ fehlt.",
+                "Kopfzeile"));
+        if (messages.Any(message => message.Severity == ImportMessageSeverity.Error))
+            return ValueTask.FromResult(new ImportParseResult<ImportedStudent>([], messages));
+
+        var students = new List<ImportedStudent>();
+        foreach (var record in records.Skip(1))
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            if (record.Fields.All(string.IsNullOrWhiteSpace)) continue;
+            var source = $"Zeile {record.LineNumber}";
+            if (record.Fields.Count != headers.Count)
+            {
+                messages.Add(new ImportMessage(
+                    ImportMessageSeverity.Error,
+                    "master-data.columns",
+                    $"Die Zeile enthält {record.Fields.Count} statt {headers.Count} Spalten.",
+                    source));
+                continue;
+            }
+
+            var gender = ParseGender(Value(record, columns, "gender"), source, messages);
+            var dateOfBirth = ParseDate(Value(record, columns, "birthDate"), source, messages);
+            var externalId = FirstNotEmpty(
+                Value(record, columns, "externKey"),
+                Value(record, columns, "id"));
+            students.Add(new ImportedStudent
+            {
+                FirstName = Value(record, columns, "foreName"),
+                LastName = Value(record, columns, "longName"),
+                Gender = gender,
+                DateOfBirth = dateOfBirth,
+                ExternalId = externalId,
+                GroupName = NullIfWhiteSpace(Value(record, columns, "klasse.name")),
+                Contact = new ImportedStudentContact
+                {
+                    Email = NullIfWhiteSpace(Value(record, columns, "address.email")),
+                    MobilePhone = NullIfWhiteSpace(Value(record, columns, "address.mobile")),
+                    Phone = NullIfWhiteSpace(Value(record, columns, "address.phone")),
+                    City = NullIfWhiteSpace(Value(record, columns, "address.city")),
+                    PostalCode = NullIfWhiteSpace(Value(record, columns, "address.postCode")),
+                    Street = NullIfWhiteSpace(Value(record, columns, "address.street")),
+                },
+                SourceReference = source,
+            });
+        }
+
+        return ValueTask.FromResult(new ImportParseResult<ImportedStudent>(students, messages));
+    }
+
+    private static Gender? ParseGender(
+        string value,
+        string source,
+        ICollection<ImportMessage> messages)
+    {
+        if (string.IsNullOrWhiteSpace(value)) return null;
+        return value.Trim().ToLowerInvariant() switch
+        {
+            "m" or "männlich" or "maennlich" => Gender.M,
+            "w" or "weiblich" => Gender.W,
+            "d" or "divers" => Gender.D,
+            _ => UnknownGender(value, source, messages),
+        };
+    }
+
+    private static Gender? UnknownGender(
+        string value,
+        string source,
+        ICollection<ImportMessage> messages)
+    {
+        messages.Add(new ImportMessage(
+            ImportMessageSeverity.Warning,
+            "master-data.gender.unknown",
+            $"Das Geschlecht „{value.Trim()}“ ist unbekannt und wird nicht übernommen.",
+            source));
+        return null;
+    }
+
+    private static DateOnly? ParseDate(
+        string value,
+        string source,
+        ICollection<ImportMessage> messages)
+    {
+        if (string.IsNullOrWhiteSpace(value)) return null;
+        if (DateOnly.TryParseExact(
+                value.Trim(),
+                "dd.MM.yyyy",
+                CultureInfo.InvariantCulture,
+                DateTimeStyles.None,
+                out var date))
+            return date;
+
+        messages.Add(new ImportMessage(
+            ImportMessageSeverity.Error,
+            "master-data.birth-date.invalid",
+            $"Das Geburtsdatum „{value.Trim()}“ besitzt nicht das erwartete Format TT.MM.JJJJ.",
+            source));
+        return null;
+    }
+
+    private static string Value(
+        DelimitedRecord record,
+        IReadOnlyDictionary<string, int> columns,
+        string column) =>
+        columns.TryGetValue(column, out var index) && index < record.Fields.Count
+            ? record.Fields[index].Trim()
+            : "";
+
+    private static string? FirstNotEmpty(params string[] values) =>
+        values.Select(NullIfWhiteSpace).FirstOrDefault(value => value is not null);
+
+    private static string? NullIfWhiteSpace(string? value) =>
+        string.IsNullOrWhiteSpace(value) ? null : value.Trim();
+
+}

LehrerApp.Core/Interfaces/IRepositories.cs

@@ -25,6 +25,9 @@ public interface ISeatingPlanRepository
     List<SeatingPlan> GetByGroup(Guid groupId);
     void Save(SeatingPlan plan);
     void Delete(Guid id);
+    /// <summary>Stellt einen über <see cref="Delete"/> in den Papierkorb verschobenen Sitzplan
+    /// wieder her (14.3). Tut nichts, falls der Papierkorb-Eintrag nicht (mehr) existiert.</summary>
+    void Restore(Guid trashId);
 }
 public interface IGroupMembershipRepository
 {
@@ -57,6 +60,8 @@ public interface IGradingKeyTemplateRepository
     GradingKeyTemplate? GetById(Guid id);
     void Save(GradingKeyTemplate template);
     void Delete(Guid id);
+    /// <summary>Siehe ISeatingPlanRepository.Restore (14.3).</summary>
+    void Restore(Guid trashId);
 }
 public interface IGradeRepository
 {
@@ -64,6 +69,8 @@ public interface IGradeRepository
     List<Grade> GetByGroup(Guid groupId);
     void Save(Grade grade);
     void Delete(Guid id);
+    /// <summary>Siehe ISeatingPlanRepository.Restore (14.3).</summary>
+    void Restore(Guid trashId);
 }
 public interface IGradingSchemeRepository
 {
@@ -117,9 +124,70 @@ public interface ISubstitutionEntryRepository
 {
     List<SubstitutionEntry> GetAll();
     List<SubstitutionEntry> GetByDate(DateOnly date);
+    /// Für den WebUntis-Abgleich (idempotentes Update statt Duplikat je iCal-UID).
+    SubstitutionEntry? GetByExternalId(string externalId);
     void Save(SubstitutionEntry entry);
     void Delete(Guid id);
 }
+/// Zuletzt bekannter Zustand aller WebUntis-iCal-Termine, für den Abgleich bei jedem Abruf.
+public interface IUntisSnapshotRepository
+{
+    List<UntisSnapshotEntry> GetAll();
+    void Save(UntisSnapshotEntry entry);
+    void Delete(Guid id);
+}
+/// Lokaler Cache der Fehlzeiten-Berichtszeilen fürs Klassenlehrer-Feature (siehe TODO.md) —
+/// bewusst nicht synchronisiert (siehe Implementierung), jedes Gerät ruft WebUntis selbst ab.
+public interface IUntisAbsenceCacheRepository
+{
+    List<UntisAbsenceCacheEntry> GetByClassAndRange(string className, int startDate, int endDate);
+    /// Löscht vorhandene Zeilen im Bereich und fügt die übergebenen neu ein — fürs "heiße" Fenster,
+    /// dessen Inhalt sich seit dem letzten Abruf geändert haben kann.
+    void ReplaceRange(string className, int startDate, int endDate, IEnumerable<UntisAbsenceCacheEntry> entries);
+    /// Fügt nur ein, ohne zu löschen — fürs Erweitern der als endgültig angenommenen "kalten" Historie.
+    void InsertRange(IEnumerable<UntisAbsenceCacheEntry> entries);
+}
+/// Lokaler Cache der Klassenbuch-Berichtszeilen anderer Lehrkräfte fürs Klassenlehrer-Feature —
+/// gleiches Muster wie <see cref="IUntisAbsenceCacheRepository"/>.
+public interface IUntisClassRegisterCacheRepository
+{
+    List<UntisClassRegisterCacheEntry> GetByClassAndRange(string className, int startDate, int endDate);
+    void ReplaceRange(string className, int startDate, int endDate, IEnumerable<UntisClassRegisterCacheEntry> entries);
+    void InsertRange(IEnumerable<UntisClassRegisterCacheEntry> entries);
+}
+/// Lokaler Cache der aktuellen Klassenliste (Roster) fürs Klassenlehrer-Feature — anders als
+/// Fehlzeiten/Klassenbuch keine Historie, deshalb kein ReplaceRange/InsertRange, sondern immer der
+/// komplette Ersatz des zuletzt bekannten Standes einer Klasse.
+public interface IUntisStudentRosterCacheRepository
+{
+    List<UntisStudentRosterCacheEntry> GetByClass(string className);
+    void ReplaceAll(string className, IEnumerable<UntisStudentRosterCacheEntry> entries);
+}
+/// Merkt sich je Klasse+Berichtsart, wann das heiße Fenster zuletzt aufgefrischt wurde und wie weit
+/// die kalte Historie bereits abgedeckt ist (siehe UntisReportCacheService).
+public interface IUntisCacheFetchStateRepository
+{
+    UntisCacheFetchState? Get(string className, UntisCacheKind kind);
+    void Save(UntisCacheFetchState state);
+}
+/// Vom Nutzer bestätigte Zuordnungen WebUntis-Wochenmuster → LearningGroup.
+public interface IUntisSlotMappingRepository
+{
+    List<UntisSlotMapping> GetAll();
+    void Save(UntisSlotMapping mapping);
+    void Delete(Guid id);
+}
+/// Lokal importierter, rein informativer Schuljahresplan. Bewusst getrennt von WebUntis-
+/// Stundenplanabweichungen und nicht über den App-Sync repliziert; jedes Gerät bezieht denselben
+/// externen Feed selbst.
+public interface IAnnualPlanEventRepository
+{
+    List<AnnualPlanEvent> GetAll();
+    List<AnnualPlanEvent> GetByRange(DateOnly from, DateOnly to);
+    AnnualPlanEvent? GetByExternalId(string externalId);
+    void Save(AnnualPlanEvent entry);
+    void Delete(Guid id);
+}
 public interface IDocumentationRepository
 {
     List<Documentation> GetByStudent(Guid studentId);
@@ -136,8 +204,12 @@ public interface IWorkTaskRepository
 {
     List<WorkTask> GetByStatus(WorkTaskStatus status);
     List<WorkTask> GetAll();
+    /// Für das "Anstehende Aufgaben"-Widget im Kurs-Dashboard (9.2/7.2).
+    List<WorkTask> GetByGroup(Guid groupId);
     void Save(WorkTask task);
     void Delete(Guid id);
+    /// <summary>Siehe ISeatingPlanRepository.Restore (14.3).</summary>
+    void Restore(Guid trashId);
 }
 public interface ITimeEntryRepository
 {
@@ -146,6 +218,8 @@ public interface ITimeEntryRepository
     List<TimeEntry> GetByTask(Guid taskId);
     void Save(TimeEntry entry);
     void Delete(Guid id);
+    /// <summary>Siehe ISeatingPlanRepository.Restore (14.3).</summary>
+    void Restore(Guid trashId);
 }
 public interface IParticipationSessionRepository
 {
@@ -209,3 +283,11 @@ public interface IAlternativeLessonPathRepository
     void Save(AlternativeLessonPath path);
     void Delete(Guid id);
 }
+/// <summary>Papierkorb (14.3) — reine Übersicht/Aufräumen; das eigentliche Wiederherstellen läuft
+/// über die jeweilige Fach-Repository (z.B. IGradeRepository.Restore), da nur die kennt, wie ein
+/// Schnappschuss ihres Modells korrekt gespeichert wird (Validierung, OnChange-Ereignis).</summary>
+public interface ITrashRepository
+{
+    List<TrashedItem> GetAll();
+    void PurgeOlderThan(DateTime cutoffUtc);
+}

LehrerApp.Core/Models/AnnualPlan.cs

@@ -0,0 +1,37 @@
+namespace LehrerApp.Core.Models;
+
+/// <summary>
+/// Ein aus dem schulweiten Jahresplan importierter Termin. Anders als <see cref="TimetableSlot"/>
+/// und <see cref="SubstitutionEntry"/> ist er reine Kalenderinformation: Er verändert weder den
+/// regulären Stundenplan noch erzeugt er Vertretungen oder Ausfälle.
+/// </summary>
+public class AnnualPlanEvent
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+
+    /// <summary>Stabile UID aus dem iCal-Feed; fachlicher Schlüssel für idempotente Importe.</summary>
+    public string ExternalId { get; set; } = "";
+
+    public string Title { get; set; } = "";
+    public string Description { get; set; } = "";
+    public string Location { get; set; } = "";
+
+    /// <summary>Erster beziehungsweise letzter tatsächlich belegter lokaler Kalendertag.</summary>
+    public DateOnly StartDate { get; set; }
+    public DateOnly EndDate { get; set; }
+
+    /// <summary>
+    /// Bei Ganztagsterminen null. Zeitgebundene UTC-Werte aus dem Feed werden beim Import in die
+    /// lokale Schulzeit Europe/Berlin umgerechnet.
+    /// </summary>
+    public TimeOnly? StartTime { get; set; }
+    public TimeOnly? EndTime { get; set; }
+    public bool IsAllDay { get; set; }
+
+    /// <summary>ClassyPlan-Gruppe, z.B. "Lehrkräfte" oder "Öffentlich".</summary>
+    public string CalendarGroup { get; set; } = "";
+    public string Color { get; set; } = "";
+    public string Status { get; set; } = "CONFIRMED";
+    public int Sequence { get; set; }
+    public DateTime? SourceLastModifiedUtc { get; set; }
+}

LehrerApp.Core/Models/Exam.cs

@@ -13,6 +13,12 @@ public class Exam
     public DateOnly? ReturnedAt { get; set; }
     public Niveau? Niveau { get; set; }
     public string? Notes { get; set; }
+    public DateOnly? ApprovalGrantedAt { get; set; }
+    public DateOnly? AnnouncedAt { get; set; }
+    /// Verknüpft Parallelklausuren (gleiche Arbeit, mehrere Kurse) für den Kurs-Umschalter
+    /// auf der Klausuren-Hauptseite — Id der "Ursprungs"-Klausur, gesetzt beim Duplizieren
+    /// (siehe ExamDialogViewModel.Save).
+    public Guid? SharedExamGroupId { get; set; }
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 }

LehrerApp.Core/Models/HazardAssessment.cs

@@ -0,0 +1,62 @@
+namespace LehrerApp.Core.Models;
+
+/// <summary>
+/// Gefährdungsbeurteilung zu einem Experiment (fachspezifisch, v.a. Chemie/Naturwissenschaften).
+/// Lebt NICHT als eigene LiteDB-Collection/Repository, sondern wird als JSON serialisiert und
+/// über die bestehende <see cref="DocumentAttachment"/>-Infrastruktur an eine <see cref="Lesson"/>
+/// angehängt (Dateiname endet auf ".gbu.json", siehe TODO.md 4.2) — das reicht für Bearbeiten,
+/// Anzeigen und PDF-Export, ohne eine weitere synchronisierte Entität samt Repository/DI-Wiring
+/// einzuführen (bewusste Scope-Entscheidung, siehe TODO.md).
+/// </summary>
+public class HazardAssessment
+{
+    public string Title { get; set; } = "";
+    /// Klassenstufe/Kurs, z.B. "10c" — vorbelegt aus der zugehörigen Lerngruppe, frei änderbar.
+    public string GroupLabel { get; set; } = "";
+    public DateOnly? Date { get; set; }
+    public ExperimentKind Kind { get; set; } = ExperimentKind.Lehrerversuch;
+    /// Kurzbeschreibung der Durchführung.
+    public string Procedure { get; set; } = "";
+    public List<HazardSubstance> Substances { get; set; } = [];
+    public List<string> Hazards { get; set; } = [];
+    public List<string> ProtectiveMeasures { get; set; } = [];
+    public string FirstAid { get; set; } = "";
+    public string Disposal { get; set; } = "";
+    public string Notes { get; set; } = "";
+    /// Ob ein KI-Entwurf beteiligt war — steuert den Rechtssicherheits-Hinweis im PDF-Export.
+    public bool IsAiAssisted { get; set; }
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+}
+
+public class HazardSubstance
+{
+    public string Name { get; set; } = "";
+    /// CAS-Nummer, falls über die Chemikalien-Recherche (TODO.md 4.2-Nachtrag) ermittelt oder von
+    /// Hand eingetragen — optional, nur zur eindeutigeren Wiedererkennung/Recherche.
+    public string Cas { get; set; } = "";
+    /// Typische Einsatzmenge, z.B. "ca. 2 g" — Freitext statt strukturierter Einheit, da der
+    /// Rahmen (fest/Lösung/Gas, Konzentration ...) zu unterschiedlich für ein starres Feld ist.
+    public string Amount { get; set; } = "";
+    public List<GhsPictogram> GhsPictograms { get; set; } = [];
+    /// "Gefahr" oder "Achtung" (GHS-Signalwort) — Freitext statt Enum, da GESTIS-Recherchen auch
+    /// mal ohne eindeutiges Signalwort zurückkommen können.
+    public string SignalWord { get; set; } = "";
+    /// H-Sätze (Gefahrenhinweise) als Freitext, z.B. "H314, H290" oder mit Kurztext — bewusst kein
+    /// eingebauter Katalog amtlicher H-/P-Satz-Texte in der App (Fehlerrisiko bei
+    /// sicherheitsrelevanten Angaben), die Lehrkraft prüft/ergänzt gegen das Sicherheitsdatenblatt.
+    public string HStatements { get; set; } = "";
+    public string PStatements { get; set; } = "";
+    /// Tätigkeitsbeschränkung laut RiSU-Stoffliste (Freigabe für alle/Schulversuche/nur
+    /// Lehrkraft, Altersgrenzen, Ersatzstoffprüfung nötig, verboten, ...) — Freitext, da die
+    /// tatsächlichen Formulierungen zu unterschiedlich für ein starres Feld/Enum sind.
+    public string ActivityRestriction { get; set; } = "";
+}
+
+public enum ExperimentKind { Lehrerversuch, Schuelerversuch, Demonstrationsversuch }
+
+/// Die neun GHS-Gefahrenpiktogramme (Global Harmonisiertes System).
+public enum GhsPictogram
+{
+    Explosive, Flammable, Oxidizing, CompressedGas, Corrosive,
+    Toxic, Harmful, HealthHazard, Environmental,
+}

LehrerApp.Core/Models/LearningGroup.cs

@@ -13,6 +13,19 @@ public class LearningGroup
     public bool IsActive { get; set; } = true;
     public bool IsOwnClass { get; set; }
     public bool IsDifferentiated { get; set; }
+    /// <summary>
+    /// Default true (alte Datensätze ohne dieses Feld verhalten sich damit unverändert wie bisher
+    /// - LiteDB liefert den C#-Default für fehlende Felder). Für Gruppen wie Klassenrat oder
+    /// Willkommenskreis, die keinen inhaltlichen Verlaufsplan brauchen, in den Stammdaten
+    /// deaktivierbar - blendet die Erinnerung "Ungeplante Stunden" im Dashboard für diese Gruppe aus.
+    /// </summary>
+    public bool RequiresLessonPlanning { get; set; } = true;
+    /// <summary>
+    /// WebUntis-interne Unterrichtsnummer (lsid) dieser Lerngruppe, Grundlage für den Abruf des
+    /// "Fehlzeiten pro Unterricht"-Berichts. Wird von WebUntis pro Schuljahr neu vergeben und muss
+    /// deshalb händisch je Lerngruppe/Schuljahr gepflegt werden - keine JSON-RPC-Methode liefert sie.
+    /// </summary>
+    public int? WebUntisLessonId { get; set; }
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 }

LehrerApp.Core/Models/Participation.cs

@@ -60,6 +60,11 @@ public enum AttendanceStatus
     OtherSchoolEvent,
     // Hinten angefügt, damit die numerischen Werte bereits gespeicherter LiteDB-Einträge stabil bleiben.
     Present,
+    Late,
+    SignificantlyLate,
+    LeftDuringClass,
+    LearningIsland,
+    Suspended,
 }
 
 /// <summary>

LehrerApp.Core/Models/Planning.cs

@@ -43,7 +43,7 @@ public class Unit
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 }
-public class Lesson
+public class Lesson : IHasAttachments
 {
     public Guid Id { get; set; } = Guid.NewGuid();
     public Guid UnitId { get; set; }
@@ -56,8 +56,18 @@ public class Lesson
     public TimeOnly? StartTime { get; set; }
     public List<LessonPhaseStep> Phases { get; set; } = [];
     public string? Homework { get; set; }
+    /// Markiert, dass die hier eingetragene Hausaufgabe in einer Folgestunde besprochen/kontrolliert
+    /// wurde — treibt das Stundenplan-Badge "Hausaufgabe kontrollieren" (4.5.4).
+    public bool HomeworkChecked { get; set; }
+    /// Bewusstes Abschalten der Erinnerung, ohne eine tatsächliche Kontrolle vorzutäuschen (z.B.
+    /// wenn die Hausaufgabe absichtlich nicht mehr kontrolliert wird).
+    public bool HomeworkCheckDismissed { get; set; }
     public string? Reflection { get; set; }
     public LessonStatus Status { get; set; } = LessonStatus.Planned;
+    /// Material/Arbeitsblätter sowie fachspezifische Anhänge (z.B. Experiment- und
+    /// Gefährdungsbeurteilungs-Dokumente im Chemieunterricht) — dieselbe Anhang-Infrastruktur wie
+    /// bei <see cref="Documentation"/>.
+    public List<DocumentAttachment> Attachments { get; set; } = [];
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 }
 
@@ -200,6 +210,12 @@ public class SubstitutionEntry
     /// Bezeichnung (bei SpecialAssignment, z.B. "Ausflug ins Museum", "Berufsmesse").
     public string Description { get; set; } = "";
     public string? Notes { get; set; }
+    /// Gesetzt, wenn dieser Eintrag automatisch aus dem WebUntis-iCal-Abgleich entstanden ist —
+    /// trägt die stabile iCal-UID des auslösenden Termins. Macht wiederholte Abgleich-Läufe
+    /// idempotent (Update statt Duplikat, siehe ISubstitutionEntryRepository.GetByExternalId) und
+    /// unterscheidet automatisch erzeugte von von Hand eingetragenen Ausnahmen. Bei Handeinträgen
+    /// (SubstitutionEntryDialogViewModel) bleibt es null.
+    public string? ExternalId { get; set; }
 }
 
 /// <summary>

LehrerApp.Core/Models/SeatingPlan.cs

@@ -12,7 +12,24 @@ public class SeatingPlan
     public string Room { get; set; } = "";
     public int Rows { get; set; } = 4;
     public int Columns { get; set; } = 4;
+    /// <summary>
+    /// Zeigt die Tafel unterhalb des Sitzrasters an. Der Standardwert <c>false</c>
+    /// erhält für bestehende Pläne die bisherige Darstellung oberhalb des Rasters.
+    /// </summary>
+    public bool IsBoardAtBottom { get; set; }
+    /// <summary>
+    /// Zusätzlicher horizontaler Abstand nach jeder Spalte, in Pixeln. Der Eintrag mit Index 0
+    /// liegt zwischen der ersten und zweiten Spalte. Fehlende Einträge bedeuten keinen Abstand.
+    /// </summary>
+    public List<double> ColumnGapWidths { get; set; } = [];
     public List<SeatAssignment> Assignments { get; set; } = [];
+    /// <summary>
+    /// Plätze, für die im Raum physisch kein Tisch existiert (z. B. eine Reihe mit einer Lücke) —
+    /// werden im Sitzplan ausgeblendet, bleiben aber Teil des Rasters, damit Reihen/Spalten
+    /// weiterhin korrekt ausgerichtet sind. Ein ausgeblendeter Platz kann nie belegt sein, siehe
+    /// SeatingPlanRepository.Save.
+    /// </summary>
+    public List<HiddenSeat> HiddenSeats { get; set; } = [];
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 }
@@ -23,3 +40,9 @@ public class SeatAssignment
     public int Column { get; set; }
     public Guid StudentId { get; set; }
 }
+
+public class HiddenSeat
+{
+    public int Row { get; set; }
+    public int Column { get; set; }
+}

LehrerApp.Core/Models/Student.cs

@@ -8,6 +8,8 @@ public class Student
     public string FullName => $"{LastName}, {FirstName}";
     public DateOnly? DateOfBirth { get; set; }
     public Gender? Gender { get; set; }
+    /// <summary>Quellsystembezogene Kennungen, z.B. für wiederholbare Stammdatenimporte.</summary>
+    public Dictionary<string, string> ExternalIds { get; set; } = [];
     public List<Contact> Contacts { get; set; } = [];
     public string? Notes { get; set; }
     public bool IsActive { get; set; } = true;
@@ -22,6 +24,7 @@ public class Contact
     /// <summary>Vollständige Anrede für Briefe, z.B. „Sehr geehrte Frau Mustermann,“.</summary>
     public string? LetterSalutation { get; set; }
     public string? Phone { get; set; }
+    public string? MobilePhone { get; set; }
     public string? Email { get; set; }
     public string? Street { get; set; }
     public string? PostalCode { get; set; }

LehrerApp.Core/Models/Trash.cs

@@ -0,0 +1,21 @@
+namespace LehrerApp.Core.Models;
+
+/// <summary>
+/// Papierkorb-Eintrag (14.3): ein gelöschter Datensatz, als vollständiger JSON-Schnappschuss
+/// aufbewahrt, damit er wiederhergestellt werden kann. Deckt bewusst nur Löschvorgänge OHNE
+/// Kaskade ab (z.B. eine einzelne Note, keine ganze Lerngruppe) — Details siehe
+/// <c>LiteDbContext.MoveToTrash</c>/<c>RestoreFromTrash</c> und TODO.md 14.3.
+/// </summary>
+public class TrashedItem
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    /// <summary>Modellname (<c>nameof(Grade)</c> etc.) — bestimmt, wie der Schnappschuss beim
+    /// Wiederherstellen deserialisiert und welche Repository-Methode aufgerufen wird.</summary>
+    public string EntityType { get; set; } = "";
+    public Guid EntityId { get; set; }
+    /// <summary>Vollständiger JSON-Schnappschuss der gelöschten Entität.</summary>
+    public string Snapshot { get; set; } = "";
+    /// <summary>Kurze, für die Papierkorb-Liste lesbare Beschreibung (z.B. "Note 2 (12.09.2026)").</summary>
+    public string Summary { get; set; } = "";
+    public DateTime DeletedAt { get; set; } = DateTime.UtcNow;
+}

LehrerApp.Core/Models/UntisCache.cs

@@ -0,0 +1,79 @@
+namespace LehrerApp.Core.Models;
+
+/// <summary>
+/// Lokal zwischengespeicherte Zeile aus dem WebUntis-Bericht "Fehlzeiten pro Schüler*in"
+/// (Klassenlehrer-Feature, siehe TODO.md) — 1:1-Abbildung von `UntisClassAbsenceEntryDto`
+/// (LehrerApp.Desktop), damit Fehlzeiten nicht bei jedem Öffnen der Ansicht neu abgerufen werden
+/// müssen. <c>Date</c> bewusst als <c>int</c> (yyyyMMdd) wie die DTOs statt <c>DateOnly</c>/
+/// <c>DateTime</c> — vermeidet die LiteDB-<c>DateTime</c>-Kind-Tücke (siehe CLAUDE.md) und ist
+/// direkt für Bereichs-Queries nutzbar.
+/// </summary>
+public class UntisAbsenceCacheEntry
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public string ClassName { get; set; } = "";
+    public string StudentName { get; set; } = "";
+    public int? ExternKey { get; set; }
+    public int Date { get; set; }
+    public int AbsentPeriods { get; set; }
+    public int AbsentMinutes { get; set; }
+    public string? TeacherUsernames { get; set; }
+    public string? Subject { get; set; }
+    public string? AbsenceReason { get; set; }
+    public string? Note { get; set; }
+    public int? EntryId { get; set; }
+    public string? HandledOn { get; set; }
+    public bool Counts { get; set; }
+    public string? ExcuseNote { get; set; }
+    public int? PeriodNumber { get; set; }
+    public string? Status { get; set; }
+    public bool CountsAsFullDay { get; set; }
+}
+
+/// <summary>Lokal zwischengespeicherte Zeile aus dem "-alle-"-Klassenbuchbericht, gefiltert auf
+/// Einträge anderer Lehrkräfte (Klassenlehrer-Feature) — 1:1-Abbildung von
+/// `UntisForeignClassRegisterEventDto` (LehrerApp.Desktop).</summary>
+public class UntisClassRegisterCacheEntry
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public string ClassName { get; set; } = "";
+    public int Date { get; set; }
+    public string? Subject { get; set; }
+    public string StudentName { get; set; } = "";
+    public string? TeacherUsername { get; set; }
+    public string? CategoryName { get; set; }
+    public string? CategoryGroup { get; set; }
+    public string? Text { get; set; }
+}
+
+/// <summary>Lokal zwischengespeicherte Zeile aus dem WebUntis-Schülerreport, eingeschränkt auf die
+/// für die Klassenlehrer-Übersicht nötigen Felder (Roster-Namen) — anders als Fehlzeiten/
+/// Klassenbuch keine Historie, sondern immer nur der zuletzt abgerufene Stand einer Klasse
+/// (siehe <see cref="UntisClassRegisterCacheEntry"/> für den Grund, warum es trotzdem ein eigenes
+/// Modell statt Wiederverwendung des vollen Schülerreport-DTOs ist: nur diese zwei Felder werden
+/// für den Roster-Abgleich tatsächlich gebraucht).</summary>
+public class UntisStudentRosterCacheEntry
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public string ClassName { get; set; } = "";
+    public int? ExternKey { get; set; }
+    public string DisplayName { get; set; } = "";
+}
+
+public enum UntisCacheKind { Absences, ClassRegister, Roster }
+
+/// <summary>
+/// Ein Datensatz pro (<see cref="ClassName"/>, <see cref="Kind"/>): wann das "heiße" Fenster
+/// (die letzten Tage, deren Status sich noch ändern kann) zuletzt aufgefrischt wurde, und wie weit
+/// die "kalte", als endgültig angenommene Historie bereits lückenlos zurückreicht. Getrennt von den
+/// Cache-Zeilen selbst, weil ein Fenster ohne Treffer (z.B. eine ereignislose Woche) sonst keine
+/// Zeile hinterließe, aus der sich "zuletzt abgerufen am…" ableiten ließe.
+/// </summary>
+public class UntisCacheFetchState
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public string ClassName { get; set; } = "";
+    public UntisCacheKind Kind { get; set; }
+    public DateTime? HotWindowFetchedAt { get; set; }
+    public int? ColdCoverageStartDate { get; set; }
+}

LehrerApp.Core/Models/UntisSync.cs

@@ -0,0 +1,69 @@
+namespace LehrerApp.Core.Models;
+
+/// <summary>
+/// Zuletzt bekannter Zustand eines einzelnen WebUntis-iCal-Termins (ein VEVENT) — die lokale
+/// "Sicherungskopie", gegen die jeder neue Abruf verglichen wird, um Änderungen zu erkennen
+/// (Nutzer-Feedback: "Abgleich mit einem lokalen Backup, um Änderungen zu finden"). <see cref="Uid"/>
+/// ist die stabile iCal-UID des Termins (pro Wochen-Slot über das ganze Schuljahr gleich) und
+/// dient als fachlicher Schlüssel für den Abgleich — bewusst kein <c>[BsonId]</c> darauf, da
+/// LehrerApp.Core absichtlich frei von LiteDB/Avalonia-Abhängigkeiten bleibt (siehe CLAUDE.md);
+/// die Suche nach <see cref="Uid"/> läuft stattdessen über eine gefilterte Repository-Abfrage.
+/// </summary>
+public class UntisSnapshotEntry
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public string Uid { get; set; } = "";
+    public DateOnly Date { get; set; }
+    public TimeOnly StartTime { get; set; }
+    public TimeOnly EndTime { get; set; }
+    public string? Summary { get; set; }
+    public string Location { get; set; } = "";
+    public string Description { get; set; } = "";
+    public string Status { get; set; } = "CONFIRMED";
+    public DateTime LastSeenAt { get; set; } = DateTime.UtcNow;
+}
+
+/// <summary>
+/// Vom Nutzer bestätigte Zuordnung eines regulären WebUntis-Wochenmusters (Wochentag + Uhrzeit +
+/// Fach-Kürzel + Klassen-Token aus der Beschreibung) zu einer bestehenden <see cref="LearningGroup"/>
+/// (<see cref="SubstitutionKind.Lesson"/>) oder — für Termine ohne Klassenbezug, z.B. Aufsichten —
+/// zu einer festen Pause (<see cref="SubstitutionKind.Supervision"/>, <see cref="AfterPeriod"/>
+/// statt <see cref="GroupId"/>/<see cref="PeriodNumber"/>; Nutzer-Feedback: "Zwei Termine sind
+/// meine Aufsichten, die nicht zugeordnet werden können [...] vom Zeitraster und von der Dauer her
+/// könnten die erfasst werden" — Aufsichten liegen in Pausen, nicht auf einem Unterrichtsstunden-
+/// Zeitraster, brauchen also einen eigenen Auflösungsweg statt PeriodNumber). Nur bestätigte
+/// (<see cref="Confirmed"/> true) Zuordnungen lösen automatisch geschriebene
+/// <see cref="SubstitutionEntry"/>-Einträge aus (Nutzer-Feedback: die erstmalige Zuordnung ist
+/// fehleranfällig — falsche Gruppe würde falsche Vertretungen erzeugen — und braucht deshalb eine
+/// Bestätigung, bevor sie aktiv wird; siehe UntisMappingReviewDialog).
+/// </summary>
+public class UntisSlotMapping
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public DayOfWeek Weekday { get; set; }
+    public TimeOnly StartTime { get; set; }
+    public string? Summary { get; set; }
+    public string ClassToken { get; set; } = "";
+    public SubstitutionKind Kind { get; set; } = SubstitutionKind.Lesson;
+    /// Nur bei <see cref="SubstitutionKind.Lesson"/> gesetzt.
+    public Guid? GroupId { get; set; }
+    /// Erste/primäre Stunde, zum Bestätigungszeitpunkt über PeriodScheduleService aufgelöst (siehe
+    /// UntisMatchingService) — hier gespeichert, damit UntisDiffService selbst framework-frei
+    /// bleibt und keine eigene Uhrzeit→Stunde-Auflösung braucht. Nur bei
+    /// <see cref="SubstitutionKind.Lesson"/> gesetzt.
+    public int? PeriodNumber { get; set; }
+    /// ALLE Stunden, die dieser WebUntis-Termin überdeckt (siehe UntisSlotMatch.CoveredPeriods) —
+    /// bei einer Doppelstunde mehr als eine. Entscheidet, welche TimetableSlots als "durch WebUntis
+    /// bestätigt" gelten (Nutzer-Feedback: "ich habe aber jetzt alles zugeordnet, und trotzdem
+    /// erhalte ich die Warnung, dass 16 Stunden ohne Untis-Zuordnung sind" — ohne diese Liste blieb
+    /// die zweite Stunde einer Doppelstunde immer "unzugeordnet", weil WebUntis dafür nur einen
+    /// einzigen, am Anfang beginnenden Termin meldet). Nur bei <see cref="SubstitutionKind.Lesson"/>
+    /// gesetzt.
+    public List<int> CoveredPeriods { get; set; } = [];
+    /// Nur bei <see cref="SubstitutionKind.Supervision"/> gesetzt — wie bei
+    /// <see cref="SupervisionDuty.AfterPeriod"/>, ebenfalls zum Bestätigungszeitpunkt aufgelöst
+    /// (die Pause direkt vor der WebUntis-Startzeit, 0 = vor der 1. Stunde).
+    public int? AfterPeriod { get; set; }
+    public bool Confirmed { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+}

LehrerApp.Core/Models/Weather.cs

@@ -0,0 +1,63 @@
+namespace LehrerApp.Core.Models;
+
+/// <summary>Vom Nutzer eingegebene Schuladresse. Die Geocodierung erfolgt ausschließlich
+/// serverseitig beim expliziten Speichern, nicht während der Eingabe.</summary>
+public class SchoolLocationRequest
+{
+    public string SchoolName { get; set; } = "";
+    public string Street { get; set; } = "";
+    public string PostalCode { get; set; } = "";
+    public string City { get; set; } = "";
+    public GermanState State { get; set; } = GermanState.NW;
+}
+
+/// <summary>Serverseitig gespeicherter Schulstandort einschließlich des bestätigbaren
+/// Geocoding-Ergebnisses. Koordinaten werden nie an den DWD als Straßenadresse übertragen.</summary>
+public sealed class SchoolLocationProfile : SchoolLocationRequest
+{
+    public double Latitude { get; set; }
+    public double Longitude { get; set; }
+    public string ResolvedAddress { get; set; } = "";
+    public DateTime UpdatedAt { get; set; }
+}
+
+public sealed class WeatherSnapshot
+{
+    public string StationId { get; set; } = "";
+    public string StationName { get; set; } = "";
+    public double StationDistanceKm { get; set; }
+    public DateTime? ForecastIssuedAt { get; set; }
+    public DateTime RetrievedAt { get; set; }
+    /// <summary>True, wenn der DWD vorübergehend nicht erreichbar war und der Server den zuletzt
+    /// erfolgreich gespeicherten Stand zurückgibt.</summary>
+    public bool IsStale { get; set; }
+    public List<WeatherForecastHour> Forecast { get; set; } = [];
+    public List<WeatherWarning> Warnings { get; set; } = [];
+}
+
+public sealed class WeatherForecastHour
+{
+    public DateTime ValidAt { get; set; }
+    public double? TemperatureC { get; set; }
+    public double? WindSpeedKmh { get; set; }
+    public double? WindGustKmh { get; set; }
+    public double? PrecipitationMm { get; set; }
+    /// <summary>Stündliche MOSMIX-Wahrscheinlichkeit für mehr als 0,1 mm Niederschlag.</summary>
+    public double? PrecipitationProbabilityPercent { get; set; }
+    /// <summary>MOSMIX-Sonnenscheindauer der letzten Stunde, in Minuten.</summary>
+    public double? SunshineDurationMinutes { get; set; }
+    public double? CloudCoverPercent { get; set; }
+    public int? WeatherCode { get; set; }
+}
+
+public sealed class WeatherWarning
+{
+    public string Identifier { get; set; } = "";
+    public string Event { get; set; } = "";
+    public string Headline { get; set; } = "";
+    public string Description { get; set; } = "";
+    public string Instruction { get; set; } = "";
+    public string Severity { get; set; } = "";
+    public DateTime? Onset { get; set; }
+    public DateTime? Expires { get; set; }
+}

LehrerApp.Core/Models/Workload.cs

@@ -1,10 +1,14 @@
 namespace LehrerApp.Core.Models;
 
-public class Documentation
+public class Documentation : IHasAttachments
 {
     public Guid Id { get; set; } = Guid.NewGuid();
     public Guid StudentId { get; set; }
     public Guid? GroupId { get; set; }
+    /// <summary>Unterrichtssitzung, in der die Beobachtung entstanden ist.</summary>
+    public Guid? ParticipationSessionId { get; set; }
+    /// <summary>Optionaler Bezug auf die konkrete geplante Stunde.</summary>
+    public Guid? LessonId { get; set; }
     public DocumentationType Type { get; set; }
     public DateOnly Date { get; set; }
     public string Title { get; set; } = "";
@@ -17,7 +21,13 @@ public class Documentation
     public List<DocumentAttachment> Attachments { get; set; } = [];
     /// Freie Labels zur Nachverfolgung, z.B. "Kritisch", "Nacharbeiten" — siehe `DocumentationTagDisplay`.
     public List<string> Tags { get; set; } = [];
+    /// <summary>Im Unterricht schnell erfasst und noch inhaltlich nachzuarbeiten.</summary>
+    public bool IsDraft { get; set; }
     public bool IsConfidential { get; set; }
+    /// <summary>Für die pädagogische Arbeit/Leistungsbewertung gedacht, aber nicht für den
+    /// WebUntis-Klassenbuch-Abgleich vorgesehen - blendet den Eintrag im Abgleichsdialog
+    /// (WebUntisDocumentationComparisonViewModel) aus dem Zwischenablage-Vorschlag aus.</summary>
+    public bool ExcludeFromWebUntisSync { get; set; }
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
     // Löschen entfernt den Eintrag nicht hart, sondern markiert ihn nur (Nachvollziehbarkeit, 5.1.4).
@@ -71,6 +81,17 @@ public class DocumentAttachment
     public long SizeBytes { get; set; }
     public DateTime UploadedAt { get; set; } = DateTime.UtcNow;
 }
+/// <summary>
+/// Implementiert von jedem Modell mit Datei-Anhängen (aktuell <see cref="Documentation"/> und
+/// <see cref="Lesson"/>). Anhang-Bytes reisen nicht im JSON-Sync-Ereignis mit, sondern als eigener
+/// Binärtransfer (siehe <c>SyncEventPublisher</c>/<c>EventApplier</c>/<c>AttachmentSyncer</c> in
+/// LehrerApp.Sync) — dieses Interface macht die Erkennung "hat dieses Modell Anhänge?" dort
+/// generisch statt hart auf einen einzelnen Modelltyp verdrahtet.
+/// </summary>
+public interface IHasAttachments
+{
+    List<DocumentAttachment> Attachments { get; }
+}
 // Hinten angefügt, damit die numerischen Werte bereits gespeicherter LiteDB-Einträge stabil bleiben.
 public enum DocumentationType { Conversation, Incident, SupportPlan, Absence, ParentCall, ParentLetter }
 public enum SupportStatus { Active, Completed, Paused }
@@ -87,9 +108,34 @@ public class WorkTask
     public string? Notes { get; set; }
     // Beim Abschließen (6.1.3) wird bei Weekly/Monthly automatisch die nächste Aufgabe erzeugt (6.1.4).
     public TaskRecurrence Recurrence { get; set; } = TaskRecurrence.None;
+    /// Nutzer-Feedback: "Aufgaben" wirkt wie reine Zeiterfassung, dabei braucht man auch schlichte
+    /// Erinnerungen ohne Zeitbezug (z.B. "morgen Ansage an die Klasse machen"). Reminder-Aufgaben
+    /// laufen über dieselbe Liste/dasselbe Dashboard-Widget, werden aber nie mit einem
+    /// TimeEntry verknüpft und fließen deshalb schon rein datenmodell-bedingt (WorkloadEvaluationViewModel
+    /// wertet nur TimeEntry aus, nicht WorkTask) nicht in die Zeitauswertung (6.3) ein.
+    public TaskKind Kind { get; set; } = TaskKind.WorkItem;
+    /// Nutzer-Feedback: pädagogische Erinnerungen ("Ansage an die Klasse", "Material einsammeln")
+    /// sollen sich je nach Dringlichkeit farblich absetzen können, unabhängig vom Fälligkeitsdatum.
+    public TaskPriority Priority { get; set; } = TaskPriority.Normal;
+    /// Optionale Abhak-Liste (Nutzer-Feedback: "Namensliste aller Kurs-Schüler" ODER freie
+    /// Teil-Punkte, wahlweise). Beides läuft über denselben Listentyp — bei aus der Kursliste
+    /// befüllten Einträgen ist StudentId gesetzt (Sprungziel zum Schüler), bei frei eingegebenen
+    /// Einträgen bleibt es leer. Als verschachtelte LiteDB-Liste statt eigener Collection/JSON-
+    /// String, siehe TODO.md-Nachtrag zu 6.1/9 — synct dadurch automatisch über den bestehenden
+    /// Sync-Unterbau mit (EventPublisher serialisiert das ganze WorkTask-Dokument je Save()).
+    public List<ChecklistItem> ChecklistItems { get; set; } = [];
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 }
+public class ChecklistItem
+{
+    public Guid Id { get; set; } = Guid.NewGuid();
+    public string Label { get; set; } = "";
+    public bool IsDone { get; set; }
+    /// Gesetzt, wenn der Eintrag aus der Kursliste befüllt wurde (Sprungziel zum Schüler);
+    /// bei frei eingegebenen Einträgen null.
+    public Guid? StudentId { get; set; }
+}
 public class TimeEntry
 {
     public Guid Id { get; set; } = Guid.NewGuid();
@@ -103,6 +149,8 @@ public class TimeEntry
     public string? Description { get; set; }
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 }
-public enum TaskCategory { Correction, Preparation, Admin, Meeting, Other }
+public enum TaskCategory { Correction, Preparation, Admin, Meeting, Other, Teaching }
 public enum WorkTaskStatus { Open, InProgress, Done }
 public enum TaskRecurrence { None, Weekly, Monthly }
+public enum TaskKind { WorkItem, Reminder }
+public enum TaskPriority { Low, Normal, High }

LehrerApp.Core/Services/AnnualPlanIcsParser.cs

@@ -0,0 +1,205 @@
+using System.Globalization;
+using System.Text;
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Services;
+
+/// <summary>
+/// Parser für den beobachteten ClassyPlan-Jahresplanexport. Im Gegensatz zum bewusst schmalen
+/// WebUntis-Parser unterstützt er insbesondere DATE-Ganztagstermine und mehrtägige Ereignisse.
+/// Er arbeitet absichtlich strikt: Ist ein VEVENT unvollständig oder ungültig, scheitert der
+/// gesamte Parse, damit ein nachgelagerter Snapshot-Abgleich keine bestehenden Termine löscht.
+/// </summary>
+public static class AnnualPlanIcsParser
+{
+    private static readonly TimeZoneInfo BerlinTimeZone =
+        TimeZoneInfo.FindSystemTimeZoneById("Europe/Berlin");
+
+    private readonly record struct Property(string Head, string Value)
+    {
+        public bool IsDate => Head.Contains("VALUE=DATE", StringComparison.OrdinalIgnoreCase);
+    }
+
+    public static List<AnnualPlanEvent> Parse(string icsText)
+    {
+        var lines = Unfold(icsText);
+        if (!lines.Any(l => l.Equals("BEGIN:VCALENDAR", StringComparison.OrdinalIgnoreCase)) ||
+            !lines.Any(l => l.Equals("END:VCALENDAR", StringComparison.OrdinalIgnoreCase)))
+            throw new FormatException("Kein vollständiger iCal-Kalender gefunden.");
+
+        var result = new List<AnnualPlanEvent>();
+        Dictionary<string, Property>? current = null;
+
+        foreach (var line in lines)
+        {
+            if (line.Equals("BEGIN:VEVENT", StringComparison.OrdinalIgnoreCase))
+            {
+                if (current is not null) throw new FormatException("Verschachteltes VEVENT gefunden.");
+                current = new Dictionary<string, Property>(StringComparer.OrdinalIgnoreCase);
+                continue;
+            }
+
+            if (line.Equals("END:VEVENT", StringComparison.OrdinalIgnoreCase))
+            {
+                if (current is null) throw new FormatException("END:VEVENT ohne BEGIN:VEVENT gefunden.");
+                result.Add(BuildEvent(current, result.Count + 1));
+                current = null;
+                continue;
+            }
+
+            if (current is null) continue;
+            var colon = line.IndexOf(':');
+            if (colon < 0) continue;
+            var head = line[..colon];
+            var separator = head.IndexOf(';');
+            var key = separator < 0 ? head : head[..separator];
+            if (key.Length > 0) current[key] = new Property(head, line[(colon + 1)..]);
+        }
+
+        if (current is not null) throw new FormatException("Nicht abgeschlossenes VEVENT gefunden.");
+        if (result.Select(e => e.ExternalId).Distinct(StringComparer.Ordinal).Count() != result.Count)
+            throw new FormatException("Der Jahresplan enthält doppelte iCal-UIDs.");
+        return result;
+    }
+
+    private static AnnualPlanEvent BuildEvent(Dictionary<string, Property> properties, int number)
+    {
+        var uid = Required(properties, "UID", number).Value.Trim();
+        if (uid.Length == 0) throw Invalid(number, "UID fehlt");
+
+        var startProperty = Required(properties, "DTSTART", number);
+        var endProperty = properties.GetValueOrDefault("DTEND");
+
+        DateOnly startDate;
+        DateOnly endDate;
+        TimeOnly? startTime;
+        TimeOnly? endTime;
+        var isAllDay = startProperty.IsDate;
+
+        if (isAllDay)
+        {
+            startDate = ParseDate(startProperty.Value, number, "DTSTART");
+            var endExclusive = endProperty == default
+                ? startDate.AddDays(1)
+                : endProperty.IsDate
+                    ? ParseDate(endProperty.Value, number, "DTEND")
+                    : throw Invalid(number, "DTSTART und DTEND verwenden unterschiedliche Werttypen");
+            if (endExclusive <= startDate) throw Invalid(number, "DTEND liegt nicht nach DTSTART");
+            endDate = endExclusive.AddDays(-1); // RFC 5545: DTEND eines Ganztagstermins ist exklusiv.
+            startTime = null;
+            endTime = null;
+        }
+        else
+        {
+            var start = ParseLocalDateTime(startProperty, number, "DTSTART");
+            var end = endProperty == default
+                ? start
+                : !endProperty.IsDate
+                    ? ParseLocalDateTime(endProperty, number, "DTEND")
+                    : throw Invalid(number, "DTSTART und DTEND verwenden unterschiedliche Werttypen");
+            if (end < start) throw Invalid(number, "DTEND liegt vor DTSTART");
+            startDate = DateOnly.FromDateTime(start);
+            endDate = DateOnly.FromDateTime(end);
+            startTime = TimeOnly.FromDateTime(start);
+            endTime = TimeOnly.FromDateTime(end);
+        }
+
+        return new AnnualPlanEvent
+        {
+            ExternalId = uid,
+            Title = Text(properties, "SUMMARY"),
+            Description = Text(properties, "DESCRIPTION"),
+            Location = Text(properties, "LOCATION"),
+            StartDate = startDate,
+            EndDate = endDate,
+            StartTime = startTime,
+            EndTime = endTime,
+            IsAllDay = isAllDay,
+            CalendarGroup = Text(properties, "X-GROUPNAME"),
+            Color = Text(properties, "X-COLOR"),
+            Status = Text(properties, "STATUS") is { Length: > 0 } status ? status : "CONFIRMED",
+            Sequence = int.TryParse(Raw(properties, "SEQUENCE"), NumberStyles.Integer,
+                CultureInfo.InvariantCulture, out var sequence) ? sequence : 0,
+            SourceLastModifiedUtc = ParseOptionalUtc(properties.GetValueOrDefault("LAST-MODIFIED"), number),
+        };
+    }
+
+    private static Property Required(Dictionary<string, Property> properties, string key, int number) =>
+        properties.TryGetValue(key, out var value) ? value : throw Invalid(number, $"{key} fehlt");
+
+    private static string Raw(Dictionary<string, Property> properties, string key) =>
+        properties.TryGetValue(key, out var value) ? value.Value : "";
+
+    private static string Text(Dictionary<string, Property> properties, string key) =>
+        Unescape(Raw(properties, key));
+
+    private static DateOnly ParseDate(string value, int number, string field)
+    {
+        if (DateOnly.TryParseExact(value, "yyyyMMdd", CultureInfo.InvariantCulture,
+                DateTimeStyles.None, out var date)) return date;
+        throw Invalid(number, $"ungültiges {field}: {value}");
+    }
+
+    private static DateTime ParseLocalDateTime(Property property, int number, string field)
+    {
+        var utc = property.Value.EndsWith('Z');
+        var value = utc ? property.Value[..^1] : property.Value;
+        var formats = new[] { "yyyyMMdd'T'HHmmss", "yyyyMMdd'T'HHmm" };
+        if (!DateTime.TryParseExact(value, formats, CultureInfo.InvariantCulture,
+                DateTimeStyles.None, out var parsed))
+            throw Invalid(number, $"ungültiges {field}: {property.Value}");
+
+        return utc
+            ? TimeZoneInfo.ConvertTimeFromUtc(DateTime.SpecifyKind(parsed, DateTimeKind.Utc), BerlinTimeZone)
+            : DateTime.SpecifyKind(parsed, DateTimeKind.Unspecified);
+    }
+
+    private static DateTime? ParseOptionalUtc(Property property, int number)
+    {
+        if (property == default || string.IsNullOrWhiteSpace(property.Value)) return null;
+        var local = ParseLocalDateTime(property, number, "LAST-MODIFIED");
+        return property.Value.EndsWith('Z')
+            ? TimeZoneInfo.ConvertTimeToUtc(local, BerlinTimeZone)
+            : TimeZoneInfo.ConvertTimeToUtc(DateTime.SpecifyKind(local, DateTimeKind.Unspecified), BerlinTimeZone);
+    }
+
+    private static List<string> Unfold(string text)
+    {
+        var result = new List<string>();
+        foreach (var line in text.Replace("\r\n", "\n").Replace("\r", "\n").Split('\n'))
+        {
+            if ((line.StartsWith(' ') || line.StartsWith('\t')) && result.Count > 0)
+                result[^1] += line[1..];
+            else
+                result.Add(line);
+        }
+        return result;
+    }
+
+    private static string Unescape(string value)
+    {
+        var result = new StringBuilder(value.Length);
+        for (var i = 0; i < value.Length; i++)
+        {
+            if (value[i] != '\\' || i + 1 >= value.Length)
+            {
+                result.Append(value[i]);
+                continue;
+            }
+
+            var next = value[++i];
+            result.Append(next switch
+            {
+                'n' or 'N' => '\n',
+                '\\' => '\\',
+                ',' => ',',
+                ';' => ';',
+                _ => next,
+            });
+        }
+        return result.ToString();
+    }
+
+    private static FormatException Invalid(int number, string message) =>
+        new($"Ungültiger Jahresplan-Termin #{number}: {message}.");
+}

LehrerApp.Core/Services/AttendanceBalanceService.cs

@@ -25,10 +25,13 @@ public class AttendanceBalanceService
             .Select(e => e.Status!.Value)
             .ToList();
 
-        var present      = relevant.Count(s => s == AttendanceStatus.Present);
+        var present      = relevant.Count(s => s is AttendanceStatus.Present
+            or AttendanceStatus.Late or AttendanceStatus.SignificantlyLate);
         var excused      = relevant.Count(s => s == AttendanceStatus.Excused);
-        var unexcused    = relevant.Count(s => s is AttendanceStatus.Unexcused or AttendanceStatus.Truant);
+        var unexcused    = relevant.Count(s => s is AttendanceStatus.Unexcused
-        var schoolEvent  = relevant.Count(s => s == AttendanceStatus.OtherSchoolEvent);
+            or AttendanceStatus.Truant or AttendanceStatus.LeftDuringClass);
+        var schoolEvent  = relevant.Count(s => s is AttendanceStatus.OtherSchoolEvent
+            or AttendanceStatus.LearningIsland or AttendanceStatus.Suspended);
         var pending      = relevant.Count(s => s == AttendanceStatus.ExcusePending);
         var total        = relevant.Count;
 

LehrerApp.Core/Services/DashboardSettingsService.cs

@@ -0,0 +1,62 @@
+using System.Text.Json;
+
+namespace LehrerApp.Core.Services;
+
+public sealed class DashboardCardSetting
+{
+    public string Key { get; set; } = "";
+    public bool IsVisible { get; set; } = true;
+    public int Order { get; set; }
+}
+
+/// <summary>Speichert Sichtbarkeit und Reihenfolge der Dashboard-Kacheln lokal.</summary>
+public sealed class DashboardSettingsService
+{
+    public static readonly string[] DefaultCardOrder =
+    [
+        "today", "tasks", "calendar", "excuses", "upcoming",
+        "corrections", "unplanned", "alerts", "attendance", "support", "groups",
+    ];
+
+    private readonly string _configPath;
+
+    public DashboardSettingsService(string appDataPath) =>
+        _configPath = Path.Combine(appDataPath, "dashboardsettings.json");
+
+    public List<DashboardCardSetting> Load()
+    {
+        try
+        {
+            if (File.Exists(_configPath))
+            {
+                var saved = JsonSerializer.Deserialize<List<DashboardCardSetting>>(
+                    File.ReadAllText(_configPath)) ?? [];
+                var byKey = saved
+                    .Where(s => DefaultCardOrder.Contains(s.Key))
+                    .GroupBy(s => s.Key).ToDictionary(g => g.Key, g => g.First());
+                return DefaultCardOrder.Select((key, defaultOrder) => byKey.TryGetValue(key, out var item)
+                        ? new DashboardCardSetting { Key = key, IsVisible = item.IsVisible, Order = item.Order }
+                        : new DashboardCardSetting { Key = key, IsVisible = true, Order = defaultOrder })
+                    .OrderBy(s => s.Order).ThenBy(s => Array.IndexOf(DefaultCardOrder, s.Key))
+                    .Select((s, index) => new DashboardCardSetting
+                        { Key = s.Key, IsVisible = s.IsVisible, Order = index })
+                    .ToList();
+            }
+        }
+        catch { /* beschädigte Konfiguration -> Standardreihenfolge */ }
+
+        return DefaultCardOrder.Select((key, index) => new DashboardCardSetting
+            { Key = key, IsVisible = true, Order = index }).ToList();
+    }
+
+    public void Save(IEnumerable<DashboardCardSetting> settings)
+    {
+        var normalized = settings.Select((s, index) => new DashboardCardSetting
+        {
+            Key = s.Key,
+            IsVisible = s.IsVisible,
+            Order = index,
+        }).ToList();
+        File.WriteAllText(_configPath, JsonSerializer.Serialize(normalized));
+    }
+}

LehrerApp.Core/Services/ExamPriorityService.cs

@@ -0,0 +1,67 @@
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Services;
+
+/// Zählt den Korrekturfortschritt einer Klausur aus den vorhandenen Ergebnissen, statt einen
+/// eigenen "Korrektur läuft"-Status pflegen zu müssen — ein `ExamResult` gilt als erledigt, sobald
+/// entweder Punkte/Note eingetragen oder der Schüler als abwesend markiert wurde. Von
+/// DashboardViewModel (Karte "Offene Korrekturen") und ExamsOverviewViewModel (Klausuren-
+/// Hauptseite) gemeinsam genutzt, damit beide Stellen exakt dieselbe Zahl zeigen.
+public static class ExamCorrectionCounter
+{
+    public static (int Expected, int Evaluated) Count(Exam exam, List<GroupMembership> groupMemberships,
+        List<ExamResult> examResults)
+    {
+        var expected = groupMemberships.Count(m => GroupMembershipService.IsActiveOn(m, exam.Date));
+        var evaluated = examResults.Count(r => r.Absent || !string.IsNullOrWhiteSpace(r.Grade) || r.Points.Count > 0);
+        return (expected, Math.Min(evaluated, expected));
+    }
+}
+
+public enum ExamListStatus { Planned, AwaitingCorrection, CorrectionInProgress, CorrectionStuck, AwaitingReturn, Returned }
+
+public readonly record struct ExamPriorityInfo(ExamListStatus Status, double Score);
+
+/// Ordnet Klausuren für die Klausuren-Hauptseite nach einem unsichtbaren Prioritäts-Score statt
+/// nach Datum: unbearbeitete/überfällige Korrekturen oben, erledigte unten. "Korrektur läuft"
+/// wird bewusst nicht als eigener, manuell zu pflegender Status abgelegt (siehe
+/// `ExamCorrectionCounter`) — bleibt eine Klausur trotzdem lange in Bearbeitung hängen (typischer
+/// Grund: ein einzelner nie nachschreibender Schüler), fällt sie nach `StuckAfterDays` aus der
+/// dringenden Zone in eine ruhige "hängt fest"-Einstufung, statt dauerhaft oben zu kleben.
+public static class ExamPriorityService
+{
+    public const int StuckAfterDays = 21;
+    private const int CorrectionGraceDays = 3;
+
+    public static ExamPriorityInfo Evaluate(Exam exam, int expected, int evaluated, DateOnly today)
+    {
+        if (exam.Status == ExamStatus.Returned)
+            return new ExamPriorityInfo(ExamListStatus.Returned, -100);
+
+        if (exam.Status == ExamStatus.Graded)
+            return new ExamPriorityInfo(ExamListStatus.AwaitingReturn, 300);
+
+        if (exam.Status == ExamStatus.Planned)
+        {
+            var daysUntil = exam.Date.DayNumber - today.DayNumber;
+            return new ExamPriorityInfo(ExamListStatus.Planned, Math.Clamp(150 - daysUntil, 20, 150));
+        }
+
+        // Status == Conducted: Korrekturfortschritt entscheidet über die genaue Einstufung.
+        var daysSince = today.DayNumber - exam.Date.DayNumber;
+
+        if (evaluated <= 0)
+        {
+            var score = daysSince <= CorrectionGraceDays ? 400 + daysSince * 10 : 600 + daysSince;
+            return new ExamPriorityInfo(ExamListStatus.AwaitingCorrection, score);
+        }
+
+        if (expected > 0 && evaluated >= expected)
+            return new ExamPriorityInfo(ExamListStatus.AwaitingReturn, 300);
+
+        if (daysSince > StuckAfterDays)
+            return new ExamPriorityInfo(ExamListStatus.CorrectionStuck, 90);
+
+        return new ExamPriorityInfo(ExamListStatus.CorrectionInProgress, 400 + daysSince * 3);
+    }
+}

LehrerApp.Core/Services/GroupRolloverService.cs

@@ -60,6 +60,8 @@ public sealed class GroupRolloverService(
             IsActive = true,
             IsOwnClass = source.IsOwnClass,
             IsDifferentiated = source.IsDifferentiated,
+            // WebUntisLessonId bewusst nicht übernommen: WebUntis vergibt sie pro Schuljahr neu,
+            // eine übernommene alte lsid würde in der Folgegruppe stumm falsche Fehlzeiten liefern.
         };
 
         var sourceWasActive = source.IsActive;

LehrerApp.Core/Services/IcsParser.cs

@@ -0,0 +1,143 @@
+using System.Globalization;
+using System.Text;
+
+namespace LehrerApp.Core.Services;
+
+/// <summary>Ein einzelner Termin (VEVENT) aus einer geparsten iCal-Datei — z.B. aus dem
+/// WebUntis-Stundenplan-Export (TODO.md, "WebUntis-iCal-Abgleich").</summary>
+public class UntisIcsEvent
+{
+    public string Uid { get; set; } = "";
+    public DateOnly Date { get; set; }
+    public DayOfWeek Weekday => Date.DayOfWeek;
+    public TimeOnly StartTime { get; set; }
+    public TimeOnly EndTime { get; set; }
+    public string? Summary { get; set; }
+    public string Location { get; set; } = "";
+    public string Description { get; set; } = "";
+    public string Status { get; set; } = "CONFIRMED";
+}
+
+/// <summary>
+/// Bewusst kein vollständiger RFC-5545-Parser, sondern genau der Ausschnitt, den der reale
+/// WebUntis-Export tatsächlich nutzt (verifiziert an einem echten Export, siehe Planungsdokument):
+/// flache VEVENT-Liste ohne RRULE-Wiederholung, ohne Line-Folding, ohne VALARM. `DTSTART;TZID=...`
+/// wird als lokale Wanduhrzeit gelesen (die App kennt ohnehin nur "lokale Zeit" der Schule, siehe
+/// PeriodScheduleService) — ein `...Z`-Suffix (UTC) wird defensiv unterstützt, auch wenn im realen
+/// Export nicht beobachtet. Termine mit unbekanntem DTSTART-Format werden übersprungen statt die
+/// gesamte Verarbeitung abzubrechen (gleiche Fail-soft-Haltung wie EventApplier in LehrerApp.Sync).
+/// </summary>
+public static class IcsParser
+{
+    private static readonly TimeZoneInfo BerlinTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Europe/Berlin");
+
+    public static List<UntisIcsEvent> Parse(string icsText)
+    {
+        var lines = Unfold(icsText);
+        var events = new List<UntisIcsEvent>();
+        Dictionary<string, (string Params, string Value)>? current = null;
+
+        foreach (var line in lines)
+        {
+            if (line == "BEGIN:VEVENT") { current = []; continue; }
+            if (line == "END:VEVENT")
+            {
+                if (current is not null && TryBuildEvent(current, out var evt)) events.Add(evt);
+                current = null;
+                continue;
+            }
+            if (current is null) continue;
+
+            var (key, paramsPart, value) = SplitPropertyLine(line);
+            if (key.Length == 0) continue;
+            current[key] = (paramsPart, value);
+        }
+
+        return events;
+    }
+
+    // RFC 5545 Line-Folding: eine Fortsetzungszeile beginnt mit Leerzeichen/Tab und gehört zur
+    // Vorzeile — im realen WebUntis-Export nicht beobachtet (Zeilen deutlich unter dem 75-Oktett-
+    // Limit), aber defensiv unterstützt, falls sich der Export einmal ändert.
+    private static List<string> Unfold(string icsText)
+    {
+        var rawLines = icsText.Replace("\r\n", "\n").Replace("\r", "\n").Split('\n');
+        var result = new List<string>();
+        foreach (var raw in rawLines)
+        {
+            if ((raw.StartsWith(' ') || raw.StartsWith('\t')) && result.Count > 0)
+                result[^1] += raw[1..];
+            else
+                result.Add(raw);
+        }
+        return result;
+    }
+
+    // "KEY;PARAM=VAL;PARAM2=VAL2:VALUE" — der erste unescapte Doppelpunkt trennt Wert vom Rest.
+    private static (string Key, string Params, string Value) SplitPropertyLine(string line)
+    {
+        var colonIndex = line.IndexOf(':');
+        if (colonIndex < 0) return ("", "", "");
+        var head = line[..colonIndex];
+        var value = line[(colonIndex + 1)..];
+        var semiIndex = head.IndexOf(';');
+        return semiIndex < 0 ? (head, "", value) : (head[..semiIndex], head[(semiIndex + 1)..], value);
+    }
+
+    private static bool TryBuildEvent(Dictionary<string, (string Params, string Value)> props, out UntisIcsEvent evt)
+    {
+        evt = new UntisIcsEvent();
+        if (!props.TryGetValue("UID", out var uid) || string.IsNullOrWhiteSpace(uid.Value)) return false;
+        evt.Uid = uid.Value;
+
+        if (!props.TryGetValue("DTSTART", out var dtStart) || !TryParseLocalDateTime(dtStart, out var start))
+            return false;
+        evt.Date = DateOnly.FromDateTime(start);
+        evt.StartTime = TimeOnly.FromDateTime(start);
+
+        if (props.TryGetValue("DTEND", out var dtEnd) && TryParseLocalDateTime(dtEnd, out var end))
+            evt.EndTime = TimeOnly.FromDateTime(end);
+
+        evt.Summary = props.TryGetValue("SUMMARY", out var summary) && !string.IsNullOrWhiteSpace(summary.Value)
+            ? Unescape(summary.Value) : null;
+        evt.Location = props.TryGetValue("LOCATION", out var location) ? Unescape(location.Value) : "";
+        evt.Description = props.TryGetValue("DESCRIPTION", out var description) ? Unescape(description.Value) : "";
+        evt.Status = props.TryGetValue("STATUS", out var status) && !string.IsNullOrWhiteSpace(status.Value)
+            ? status.Value : "CONFIRMED";
+        return true;
+    }
+
+    // "TZID=Europe/Berlin:20260817T075000" (lokale Wanduhrzeit) oder "20260817T075000Z" (UTC).
+    private static bool TryParseLocalDateTime((string Params, string Value) prop, out DateTime local)
+    {
+        local = default;
+        var value = prop.Value;
+        var isUtc = value.EndsWith('Z');
+        var digits = isUtc ? value[..^1] : value;
+
+        if (!DateTime.TryParseExact(digits, "yyyyMMdd'T'HHmmss", CultureInfo.InvariantCulture,
+                DateTimeStyles.None, out var parsed))
+            return false;
+
+        local = isUtc
+            ? TimeZoneInfo.ConvertTimeFromUtc(DateTime.SpecifyKind(parsed, DateTimeKind.Utc), BerlinTimeZone)
+            : parsed;
+        return true;
+    }
+
+    private static string Unescape(string value)
+    {
+        var sb = new StringBuilder(value.Length);
+        for (var i = 0; i < value.Length; i++)
+        {
+            if (value[i] == '\\' && i + 1 < value.Length)
+            {
+                var next = value[i + 1];
+                sb.Append(next switch { ';' => ';', ',' => ',', 'n' or 'N' => '\n', '\\' => '\\', _ => next });
+                i++;
+            }
+            else sb.Append(value[i]);
+        }
+        return sb.ToString();
+    }
+}

LehrerApp.Core/Services/StudentImportService.cs

@@ -0,0 +1,840 @@
+using System.Globalization;
+using System.Text;
+using LehrerApp.Core.Importing;
+using LehrerApp.Core.Interfaces;
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Services;
+
+/// <summary>
+/// Orchestriert den Schülerimport ohne UI-Abhängigkeit: Format erkennen, vollständig
+/// analysieren, Konflikte beschreiben und erst nach Bestätigung speichern.
+/// </summary>
+public sealed class StudentImportService
+{
+    private const string CreateOption = "create";
+    private const string SkipOption = "skip";
+    private const string ExistingOptionPrefix = "existing:";
+    private const string GroupOptionPrefix = "group:";
+    private const string ImportedContactRelation = "Stammdaten";
+
+    private readonly ImportHandlerCatalog<ImportedStudent> _handlers;
+    private readonly IStudentRepository _students;
+    private readonly IGroupRepository _groups;
+    private readonly IGroupMembershipRepository _memberships;
+    private readonly SchoolYearService _schoolYears;
+
+    public StudentImportService(
+        IEnumerable<IImportHandler<ImportedStudent>> handlers,
+        IStudentRepository students,
+        IGroupRepository groups,
+        IGroupMembershipRepository memberships,
+        SchoolYearService schoolYears)
+    {
+        _handlers = new ImportHandlerCatalog<ImportedStudent>(handlers);
+        _students = students;
+        _groups = groups;
+        _memberships = memberships;
+        _schoolYears = schoolYears;
+    }
+
+    public IReadOnlyList<IImportHandler<ImportedStudent>> Handlers => _handlers.Handlers;
+    public IReadOnlyList<string> SupportedExtensions => _handlers.SupportedExtensions;
+
+    public async Task<StudentImportPreview> AnalyzeAsync(
+        ImportFile file,
+        CancellationToken cancellationToken = default) =>
+        await AnalyzeInternalAsync(file, null, cancellationToken).ConfigureAwait(false);
+
+    public async Task<StudentImportPreview> AnalyzeAsync(
+        ImportFile file,
+        Guid targetGroupId,
+        CancellationToken cancellationToken = default) =>
+        await AnalyzeInternalAsync(file, targetGroupId, cancellationToken).ConfigureAwait(false);
+
+    private async Task<StudentImportPreview> AnalyzeInternalAsync(
+        ImportFile file,
+        Guid? targetGroupId,
+        CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(file);
+        var detected = await _handlers.DetectAsync(file, cancellationToken).ConfigureAwait(false);
+        var parsed = await detected.Handler.ParseAsync(file, cancellationToken).ConfigureAwait(false);
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var messages = parsed.Messages.ToList();
+        var imported = parsed.Items.Select(Normalize).ToList();
+        Validate(imported, messages);
+        if (imported.Count == 0 && messages.All(message => message.Severity != ImportMessageSeverity.Error))
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "students.empty",
+                "Die Importdatei enthält keine Schülerdaten."));
+
+        var existing = _students.GetAll(includeInactive: true);
+        var groups = _groups.GetAll(includeInactive: true);
+        var groupMemberships = groups.SelectMany(group => _memberships.GetByGroup(group.Id)).ToList();
+        var targetGroup = targetGroupId is null
+            ? null
+            : groups.SingleOrDefault(group => group.Id == targetGroupId.Value);
+        if (targetGroupId is not null && targetGroup is not { IsActive: true })
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "student.target-group.unavailable",
+                "Die Ziel-Lerngruppe existiert nicht oder ist archiviert."));
+        var currentSchoolYear = _schoolYears.CurrentSchoolYear();
+        var currentGroups = groups
+            .Where(group => group.IsActive && group.SchoolYear == currentSchoolYear)
+            .OrderBy(group => group.Name, StringComparer.OrdinalIgnoreCase)
+            .ToList();
+        var entries = new List<StudentImportEntry>();
+        var conflicts = new List<ImportConflict>();
+
+        for (var index = 0; index < imported.Count; index++)
+        {
+            var candidate = imported[index];
+            var entryId = $"student:{index + 1}";
+            var externalMatches = FindByExternalId(existing, detected.Handler.FormatId, candidate.ExternalId);
+            if (externalMatches.Count == 1)
+            {
+                var matched = externalMatches[0];
+                AddDifferenceWarning(candidate, matched, messages);
+                entries.Add(new StudentImportEntry(
+                    entryId,
+                    candidate,
+                    new StudentImportResolution(StudentImportResolutionKind.UseExisting, matched.Id),
+                    null,
+                    FieldsToSupplement(matched, candidate, detected.Handler.FormatId)));
+                continue;
+            }
+            if (externalMatches.Count > 1)
+            {
+                messages.Add(new ImportMessage(
+                    ImportMessageSeverity.Error,
+                    "student.external-id.ambiguous",
+                    "Die externe Schüler-ID ist mehreren vorhandenen Schülern zugeordnet.",
+                    candidate.SourceReference));
+                entries.Add(new StudentImportEntry(
+                    entryId,
+                    candidate,
+                    new StudentImportResolution(StudentImportResolutionKind.Skip),
+                    null,
+                    []));
+                continue;
+            }
+
+            var sameName = existing.Where(student => NamesEqual(student, candidate)).ToList();
+            var exact = candidate.DateOfBirth is not null
+                ? sameName.Where(student => student.DateOfBirth == candidate.DateOfBirth).ToList()
+                : [];
+
+            if (candidate.DateOfBirth is null && exact.Count == 0 && targetGroup is not null)
+            {
+                var memberIds = groupMemberships
+                    .Where(membership => membership.GroupId == targetGroup.Id)
+                    .Select(membership => membership.StudentId)
+                    .ToHashSet();
+                var memberMatches = sameName.Where(student => memberIds.Contains(student.Id)).ToList();
+                if (memberMatches.Count == 1)
+                    exact = memberMatches;
+            }
+
+            if (exact.Count == 1)
+            {
+                AddDifferenceWarning(candidate, exact[0], messages);
+                entries.Add(new StudentImportEntry(
+                    entryId,
+                    candidate,
+                    new StudentImportResolution(StudentImportResolutionKind.UseExisting, exact[0].Id),
+                    null,
+                    FieldsToSupplement(exact[0], candidate, detected.Handler.FormatId)));
+                continue;
+            }
+
+            if (sameName.Count == 0)
+            {
+                entries.Add(new StudentImportEntry(
+                    entryId,
+                    candidate,
+                    new StudentImportResolution(StudentImportResolutionKind.CreateNew),
+                    null,
+                    []));
+                continue;
+            }
+
+            var conflictId = $"student-match:{index + 1}";
+            conflicts.Add(CreateMatchConflict(
+                conflictId,
+                candidate,
+                sameName,
+                detected.Handler.FormatId));
+            entries.Add(new StudentImportEntry(entryId, candidate, null, conflictId, []));
+        }
+
+        var groupAssignments = targetGroup is { IsActive: true }
+            ? [new StudentImportGroupAssignment(targetGroup.Name, targetGroup.Id, null)]
+            : targetGroupId is null
+                ? BuildGroupAssignments(imported, currentGroups, conflicts, messages)
+                : [];
+
+        return new StudentImportPreview(
+            detected.Handler.FormatId,
+            detected.Handler.DisplayName,
+            entries,
+            messages,
+            conflicts,
+            groupAssignments,
+            targetGroupId,
+            Fingerprint(existing),
+            GroupStateFingerprint(groups, groupMemberships, currentSchoolYear));
+    }
+
+    public Task<StudentImportApplyResult> ApplyAsync(
+        StudentImportPreview preview,
+        IEnumerable<ImportDecision>? decisions = null,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(preview);
+        cancellationToken.ThrowIfCancellationRequested();
+        if (!preview.CanApply)
+            throw new InvalidOperationException(
+                "Der Import enthält Fehler oder keine Schüler und kann nicht angewendet werden.");
+
+        var existing = _students.GetAll(includeInactive: true);
+        if (!string.Equals(
+                preview.ExistingStudentsFingerprint,
+                Fingerprint(existing),
+                StringComparison.Ordinal))
+            throw new InvalidOperationException(
+                "Die Schülerdaten wurden seit der Vorschau verändert. Bitte analysiere die Datei erneut.");
+
+        var groups = _groups.GetAll(includeInactive: true);
+        var groupMemberships = groups.SelectMany(group => _memberships.GetByGroup(group.Id)).ToList();
+        if (!string.Equals(
+                preview.GroupStateFingerprint,
+                GroupStateFingerprint(groups, groupMemberships, _schoolYears.CurrentSchoolYear()),
+                StringComparison.Ordinal))
+            throw new InvalidOperationException(
+                "Die Lerngruppen oder Zuordnungen wurden seit der Vorschau verändert. "
+                + "Bitte analysiere die Datei erneut.");
+
+        var decisionMap = BuildDecisionMap(preview, decisions ?? []);
+        var resolutions = preview.Entries
+            .Select(entry => Resolve(entry, decisionMap))
+            .ToList();
+
+        // Alle Entscheidungen werden vor dem ersten Schreibzugriff geprüft.
+        ValidateResolutions(resolutions, existing);
+        var groupResolutions = preview.GroupAssignments.ToDictionary(
+            assignment => assignment.SourceGroupName,
+            assignment => ResolveGroup(assignment, decisionMap),
+            StringComparer.OrdinalIgnoreCase);
+        ValidateGroupResolutions(groupResolutions.Values, groups);
+
+        var createdIds = new List<Guid>();
+        var studentIdsByEntry = new Dictionary<string, Guid?>(StringComparer.Ordinal);
+        var existingById = existing.ToDictionary(student => student.Id);
+        var updated = 0;
+        var matched = 0;
+        var skipped = 0;
+        foreach (var (entry, resolution) in preview.Entries.Zip(resolutions))
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            switch (resolution.Kind)
+            {
+                case StudentImportResolutionKind.CreateNew:
+                    var created = CreateStudent(entry.Student, preview.FormatId);
+                    _students.Save(created);
+                    createdIds.Add(created.Id);
+                    studentIdsByEntry[entry.Id] = created.Id;
+                    break;
+                case StudentImportResolutionKind.UseExisting:
+                    var existingStudent = existingById[resolution.ExistingStudentId!.Value];
+                    if (Supplement(existingStudent, entry.Student, preview.FormatId))
+                    {
+                        _students.Save(existingStudent);
+                        updated++;
+                    }
+                    matched++;
+                    studentIdsByEntry[entry.Id] = existingStudent.Id;
+                    break;
+                case StudentImportResolutionKind.Skip:
+                    skipped++;
+                    studentIdsByEntry[entry.Id] = null;
+                    break;
+                default:
+                    throw new InvalidOperationException("Unbekannte Importentscheidung.");
+            }
+        }
+
+        var createdMemberships = 0;
+        foreach (var entry in preview.Entries)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            if (!studentIdsByEntry.TryGetValue(entry.Id, out var studentId)
+                || studentId is null
+                || !TryResolveTargetGroup(preview, entry, groupResolutions, out var groupId)
+                || groupId is null
+                || _memberships.GetByStudentAndGroup(studentId.Value, groupId.Value) is not null)
+                continue;
+
+            _memberships.Save(new GroupMembership
+            {
+                StudentId = studentId.Value,
+                GroupId = groupId.Value,
+                AddedOn = DateOnly.FromDateTime(DateTime.Today),
+                Period = MembershipPeriod.FullYear,
+            });
+            createdMemberships++;
+        }
+
+        return Task.FromResult(new StudentImportApplyResult(
+            createdIds.Count,
+            updated,
+            matched,
+            skipped,
+            createdMemberships,
+            createdIds.AsReadOnly()));
+    }
+
+    private static bool TryResolveTargetGroup(
+        StudentImportPreview preview,
+        StudentImportEntry entry,
+        IReadOnlyDictionary<string, Guid?> groupResolutions,
+        out Guid? groupId)
+    {
+        if (preview.TargetGroupId is not null)
+        {
+            groupId = preview.TargetGroupId;
+            return true;
+        }
+        if (entry.Student.GroupName is { } sourceGroup
+            && groupResolutions.TryGetValue(sourceGroup, out groupId))
+            return true;
+        groupId = null;
+        return false;
+    }
+
+    private static ImportedStudent Normalize(ImportedStudent student) => student with
+    {
+        FirstName = student.FirstName?.Trim() ?? "",
+        LastName = student.LastName?.Trim() ?? "",
+        ExternalId = NullIfWhiteSpace(student.ExternalId),
+        GroupName = NullIfWhiteSpace(student.GroupName),
+        Contact = Normalize(student.Contact),
+        SourceReference = NullIfWhiteSpace(student.SourceReference),
+    };
+
+    private static ImportedStudentContact? Normalize(ImportedStudentContact? contact)
+    {
+        if (contact is null) return null;
+        var normalized = contact with
+        {
+            Email = NullIfWhiteSpace(contact.Email),
+            MobilePhone = NullIfWhiteSpace(contact.MobilePhone),
+            Phone = NullIfWhiteSpace(contact.Phone),
+            Street = NullIfWhiteSpace(contact.Street),
+            PostalCode = NullIfWhiteSpace(contact.PostalCode),
+            City = NullIfWhiteSpace(contact.City),
+        };
+        return normalized.HasData ? normalized : null;
+    }
+
+    private static string? NullIfWhiteSpace(string? value) =>
+        string.IsNullOrWhiteSpace(value) ? null : value.Trim();
+
+    private static void Validate(
+        IReadOnlyList<ImportedStudent> imported,
+        ICollection<ImportMessage> messages)
+    {
+        for (var index = 0; index < imported.Count; index++)
+        {
+            var student = imported[index];
+            var source = student.SourceReference ?? $"Eintrag {index + 1}";
+            if (student.FirstName.Length == 0)
+                messages.Add(new ImportMessage(
+                    ImportMessageSeverity.Error,
+                    "student.first-name.required",
+                    "Der Vorname fehlt.",
+                    source));
+            if (student.LastName.Length == 0)
+                messages.Add(new ImportMessage(
+                    ImportMessageSeverity.Error,
+                    "student.last-name.required",
+                    "Der Nachname fehlt.",
+                    source));
+        }
+
+        var duplicateExternalIds = imported
+            .Where(student => student.ExternalId is not null)
+            .GroupBy(student => student.ExternalId!, StringComparer.OrdinalIgnoreCase)
+            .Where(group => group.Count() > 1);
+        foreach (var duplicate in duplicateExternalIds)
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "student.external-id.duplicate",
+                $"Die externe Schüler-ID „{duplicate.Key}“ kommt mehrfach vor."));
+
+        var duplicateIdentities = imported
+            .Where(student => student.FirstName.Length > 0
+                              && student.LastName.Length > 0
+                              && student.DateOfBirth is not null)
+            .GroupBy(student =>
+                $"{student.FirstName.ToUpperInvariant()}\u001f{student.LastName.ToUpperInvariant()}\u001f{student.DateOfBirth:yyyyMMdd}")
+            .Where(group => group.Count() > 1);
+        foreach (var duplicate in duplicateIdentities)
+        {
+            var student = duplicate.First();
+            messages.Add(new ImportMessage(
+                ImportMessageSeverity.Error,
+                "student.duplicate",
+                $"„{student.LastName}, {student.FirstName}“ mit Geburtsdatum "
+                + $"{DateDisplay(student.DateOfBirth)} kommt mehrfach im Import vor."));
+        }
+    }
+
+    private static ImportConflict CreateMatchConflict(
+        string conflictId,
+        ImportedStudent imported,
+        IReadOnlyList<Student> candidates,
+        ImportFormatId formatId)
+    {
+        var options = candidates.Select(candidate => new ImportConflictOption(
+                ExistingOption(candidate.Id),
+                $"Vorhandenen Schüler „{candidate.FullName}“ verwenden",
+                SupplementDescription(candidate, imported, formatId)))
+            .Append(new ImportConflictOption(
+                CreateOption,
+                "Als neuen Schüler anlegen",
+                "Der vorhandene Datensatz bleibt unverändert."))
+            .Append(new ImportConflictOption(
+                SkipOption,
+                "Eintrag überspringen"))
+            .ToList()
+            .AsReadOnly();
+
+        return new ImportConflict(
+            conflictId,
+            "Möglicher vorhandener Schüler",
+            "Vor- und Nachname stimmen überein, der Datensatz konnte aber nicht eindeutig zugeordnet werden.",
+            ImportedDetails(imported),
+            string.Join(Environment.NewLine, candidates.Select(StudentDetails)),
+            options,
+            SkipOption,
+            imported.SourceReference);
+    }
+
+    private static IReadOnlyList<Student> FindByExternalId(
+        IEnumerable<Student> existing,
+        ImportFormatId formatId,
+        string? externalId)
+    {
+        if (externalId is null) return [];
+        return existing.Where(student =>
+                student.ExternalIds is not null
+                && student.ExternalIds.TryGetValue(formatId.Value, out var stored)
+                && string.Equals(stored, externalId, StringComparison.OrdinalIgnoreCase))
+            .ToList();
+    }
+
+    private static void AddDifferenceWarning(
+        ImportedStudent imported,
+        Student existing,
+        ICollection<ImportMessage> messages)
+    {
+        var differences = new List<string>();
+        if (existing.DateOfBirth is not null && imported.DateOfBirth is not null
+            && existing.DateOfBirth != imported.DateOfBirth)
+            differences.Add("Geburtsdatum");
+        if (existing.Gender is not null && imported.Gender is not null
+            && existing.Gender != imported.Gender)
+            differences.Add("Geschlecht");
+        if (differences.Count == 0) return;
+
+        messages.Add(new ImportMessage(
+            ImportMessageSeverity.Warning,
+            "student.existing-values-kept",
+            $"Vorhandene abweichende Werte ({string.Join(", ", differences)}) werden nicht überschrieben.",
+            imported.SourceReference));
+    }
+
+    private static IReadOnlyList<string> FieldsToSupplement(
+        Student existing,
+        ImportedStudent imported,
+        ImportFormatId formatId)
+    {
+        var fields = new List<string>();
+        if (existing.DateOfBirth is null && imported.DateOfBirth is not null)
+            fields.Add("Geburtsdatum");
+        if (existing.Gender is null && imported.Gender is not null)
+            fields.Add("Geschlecht");
+        if (imported.ExternalId is not null
+            && (existing.ExternalIds is null || !existing.ExternalIds.ContainsKey(formatId.Value)))
+            fields.Add("externe Kennung");
+
+        if (imported.Contact is { HasData: true } contact)
+        {
+            var target = existing.Contacts?.FirstOrDefault(IsImportedContact);
+            AddMissingField(fields, "E-Mail", target?.Email, contact.Email);
+            AddMissingField(fields, "Mobiltelefon", target?.MobilePhone, contact.MobilePhone);
+            AddMissingField(fields, "Telefon", target?.Phone, contact.Phone);
+            AddMissingField(fields, "Straße", target?.Street, contact.Street);
+            AddMissingField(fields, "Postleitzahl", target?.PostalCode, contact.PostalCode);
+            AddMissingField(fields, "Ort", target?.City, contact.City);
+        }
+        return fields.AsReadOnly();
+    }
+
+    private static void AddMissingField(
+        ICollection<string> fields,
+        string label,
+        string? existing,
+        string? imported)
+    {
+        if (string.IsNullOrWhiteSpace(existing) && !string.IsNullOrWhiteSpace(imported))
+            fields.Add(label);
+    }
+
+    private static string SupplementDescription(
+        Student existing,
+        ImportedStudent imported,
+        ImportFormatId formatId)
+    {
+        var supplements = FieldsToSupplement(existing, imported, formatId);
+        return supplements.Count == 0
+            ? $"{StudentDetails(existing)} · keine Ergänzungen erforderlich"
+            : $"{StudentDetails(existing)} · ergänzt: {string.Join(", ", supplements)}";
+    }
+
+    private static List<StudentImportGroupAssignment> BuildGroupAssignments(
+        IEnumerable<ImportedStudent> imported,
+        IReadOnlyList<LearningGroup> currentGroups,
+        ICollection<ImportConflict> conflicts,
+        ICollection<ImportMessage> messages)
+    {
+        var result = new List<StudentImportGroupAssignment>();
+        var sourceGroups = imported
+            .Select(student => student.GroupName)
+            .Where(group => group is not null)
+            .Select(group => group!)
+            .Distinct(StringComparer.OrdinalIgnoreCase)
+            .OrderBy(group => group, StringComparer.OrdinalIgnoreCase);
+
+        foreach (var sourceGroup in sourceGroups)
+        {
+            var sourceGradeLevel = LeadingGradeLevel(sourceGroup);
+            var exact = currentGroups
+                .Where(group => string.Equals(
+                                    group.Name.Trim(), sourceGroup, StringComparison.OrdinalIgnoreCase)
+                                && group.Type == GroupType.Class
+                                && (sourceGradeLevel is null || group.GradeLevel == sourceGradeLevel))
+                .ToList();
+            var ownClassMatches = exact.Where(group => group.IsOwnClass).ToList();
+            if (ownClassMatches.Count == 1)
+                exact = ownClassMatches;
+            if (exact.Count == 1)
+            {
+                result.Add(new StudentImportGroupAssignment(sourceGroup, exact[0].Id, null));
+                continue;
+            }
+
+            var candidates = exact.Count > 1 ? exact : currentGroups;
+            if (candidates.Count == 0)
+            {
+                messages.Add(new ImportMessage(
+                    ImportMessageSeverity.Warning,
+                    "student.group.not-found",
+                    $"Für die Klasse „{sourceGroup}“ gibt es im aktuellen Schuljahr keine aktive Lerngruppe; "
+                    + "die Schüler werden ohne Gruppenzuordnung importiert."));
+                result.Add(new StudentImportGroupAssignment(sourceGroup, null, null));
+                continue;
+            }
+
+            var conflictId = $"student-group:{sourceGroup.ToLowerInvariant()}";
+            var options = candidates.Select(group => new ImportConflictOption(
+                    GroupOption(group.Id),
+                    $"Lerngruppe „{group.Name}“ verwenden",
+                    $"{group.SchoolYear} · Klassenstufe {group.GradeLevel}"))
+                .Append(new ImportConflictOption(
+                    SkipOption,
+                    "Keine Lerngruppe zuordnen"))
+                .ToList()
+                .AsReadOnly();
+            conflicts.Add(new ImportConflict(
+                conflictId,
+                "Lerngruppe zuordnen",
+                exact.Count > 1
+                    ? $"Für „{sourceGroup}“ wurden mehrere namensgleiche Lerngruppen gefunden."
+                    : $"Für „{sourceGroup}“ wurde keine eindeutig namensgleiche Lerngruppe gefunden.",
+                sourceGroup,
+                null,
+                options,
+                SkipOption));
+            result.Add(new StudentImportGroupAssignment(sourceGroup, null, conflictId));
+        }
+        return result;
+    }
+
+    private static int? LeadingGradeLevel(string groupName)
+    {
+        var digits = new string(groupName.Trim().TakeWhile(char.IsDigit).ToArray());
+        return int.TryParse(digits, CultureInfo.InvariantCulture, out var gradeLevel)
+            ? gradeLevel
+            : null;
+    }
+
+    private static Dictionary<string, ImportDecision> BuildDecisionMap(
+        StudentImportPreview preview,
+        IEnumerable<ImportDecision> decisions)
+    {
+        var conflicts = preview.Conflicts.ToDictionary(conflict => conflict.Id, StringComparer.Ordinal);
+        var map = new Dictionary<string, ImportDecision>(StringComparer.Ordinal);
+        foreach (var decision in decisions)
+        {
+            if (!conflicts.TryGetValue(decision.ConflictId, out var conflict))
+                throw new InvalidOperationException(
+                    $"Die Entscheidung verweist auf den unbekannten Konflikt „{decision.ConflictId}“.");
+            if (!conflict.Options.Any(option => option.Id == decision.SelectedOptionId))
+                throw new InvalidOperationException(
+                    $"Die Option „{decision.SelectedOptionId}“ gehört nicht zum Konflikt „{decision.ConflictId}“.");
+            if (!map.TryAdd(decision.ConflictId, decision))
+                throw new InvalidOperationException(
+                    $"Für den Konflikt „{decision.ConflictId}“ wurden mehrere Entscheidungen übergeben.");
+        }
+
+        var missing = conflicts.Keys.Where(id => !map.ContainsKey(id)).ToList();
+        if (missing.Count > 0)
+            throw new InvalidOperationException(
+                "Vor dem Import müssen alle Konflikte entschieden werden.");
+        return map;
+    }
+
+    private static StudentImportResolution Resolve(
+        StudentImportEntry entry,
+        IReadOnlyDictionary<string, ImportDecision> decisions)
+    {
+        if (entry.AutomaticResolution is not null)
+            return entry.AutomaticResolution;
+        if (entry.ConflictId is null || !decisions.TryGetValue(entry.ConflictId, out var decision))
+            throw new InvalidOperationException("Für einen Importkonflikt fehlt die Entscheidung.");
+
+        if (decision.SelectedOptionId == CreateOption)
+            return new StudentImportResolution(StudentImportResolutionKind.CreateNew);
+        if (decision.SelectedOptionId == SkipOption)
+            return new StudentImportResolution(StudentImportResolutionKind.Skip);
+        if (TryReadExistingOption(decision.SelectedOptionId, out var existingId))
+            return new StudentImportResolution(StudentImportResolutionKind.UseExisting, existingId);
+
+        throw new InvalidOperationException(
+            $"Die Konfliktoption „{decision.SelectedOptionId}“ ist unbekannt.");
+    }
+
+    private static Guid? ResolveGroup(
+        StudentImportGroupAssignment assignment,
+        IReadOnlyDictionary<string, ImportDecision> decisions)
+    {
+        if (assignment.ConflictId is null)
+            return assignment.AutomaticGroupId;
+        if (!decisions.TryGetValue(assignment.ConflictId, out var decision))
+            throw new InvalidOperationException("Für die Lerngruppenzuordnung fehlt die Entscheidung.");
+        if (decision.SelectedOptionId == SkipOption)
+            return null;
+        if (TryReadGroupOption(decision.SelectedOptionId, out var groupId))
+            return groupId;
+        throw new InvalidOperationException(
+            $"Die Gruppenoption „{decision.SelectedOptionId}“ ist unbekannt.");
+    }
+
+    private static void ValidateResolutions(
+        IReadOnlyList<StudentImportResolution> resolutions,
+        IReadOnlyCollection<Student> existing)
+    {
+        var existingIds = existing.Select(student => student.Id).ToHashSet();
+        foreach (var resolution in resolutions.Where(resolution =>
+                     resolution.Kind == StudentImportResolutionKind.UseExisting))
+        {
+            if (resolution.ExistingStudentId is not Guid id || !existingIds.Contains(id))
+                throw new InvalidOperationException(
+                    "Ein ausgewählter vorhandener Schüler existiert nicht mehr.");
+        }
+    }
+
+    private static void ValidateGroupResolutions(
+        IEnumerable<Guid?> resolutions,
+        IReadOnlyCollection<LearningGroup> groups)
+    {
+        var writableGroupIds = groups
+            .Where(group => group.IsActive)
+            .Select(group => group.Id)
+            .ToHashSet();
+        if (resolutions.Any(id => id is not null && !writableGroupIds.Contains(id.Value)))
+            throw new InvalidOperationException(
+                "Eine ausgewählte Lerngruppe existiert nicht mehr oder ist archiviert.");
+    }
+
+    private static Student CreateStudent(ImportedStudent imported, ImportFormatId formatId)
+    {
+        var student = new Student
+        {
+            FirstName = imported.FirstName,
+            LastName = imported.LastName,
+            DateOfBirth = imported.DateOfBirth,
+            Gender = imported.Gender,
+        };
+        if (imported.ExternalId is not null)
+            student.ExternalIds[formatId.Value] = imported.ExternalId;
+        var contact = CreateImportedContact(student, imported.Contact);
+        if (contact is not null) student.Contacts.Add(contact);
+        return student;
+    }
+
+    private static bool Supplement(
+        Student target,
+        ImportedStudent imported,
+        ImportFormatId formatId)
+    {
+        var changed = false;
+        if (target.DateOfBirth is null && imported.DateOfBirth is not null)
+        {
+            target.DateOfBirth = imported.DateOfBirth;
+            changed = true;
+        }
+        if (target.Gender is null && imported.Gender is not null)
+        {
+            target.Gender = imported.Gender;
+            changed = true;
+        }
+
+        target.ExternalIds ??= [];
+        if (imported.ExternalId is not null && !target.ExternalIds.ContainsKey(formatId.Value))
+        {
+            target.ExternalIds[formatId.Value] = imported.ExternalId;
+            changed = true;
+        }
+
+        if (imported.Contact is not { HasData: true } importedContact)
+            return changed;
+        target.Contacts ??= [];
+        var contact = target.Contacts.FirstOrDefault(IsImportedContact);
+        if (contact is null)
+        {
+            contact = CreateImportedContact(target, importedContact)!;
+            target.Contacts.Add(contact);
+            return true;
+        }
+
+        changed |= FillIfEmpty(contact.Email, importedContact.Email, value => contact.Email = value);
+        changed |= FillIfEmpty(contact.MobilePhone, importedContact.MobilePhone, value => contact.MobilePhone = value);
+        changed |= FillIfEmpty(contact.Phone, importedContact.Phone, value => contact.Phone = value);
+        changed |= FillIfEmpty(contact.Street, importedContact.Street, value => contact.Street = value);
+        changed |= FillIfEmpty(contact.PostalCode, importedContact.PostalCode, value => contact.PostalCode = value);
+        changed |= FillIfEmpty(contact.City, importedContact.City, value => contact.City = value);
+        return changed;
+    }
+
+    private static Contact? CreateImportedContact(
+        Student student,
+        ImportedStudentContact? imported)
+    {
+        if (imported is not { HasData: true }) return null;
+        return new Contact
+        {
+            Name = $"{student.FirstName} {student.LastName}".Trim(),
+            Relation = ImportedContactRelation,
+            Email = imported.Email,
+            MobilePhone = imported.MobilePhone,
+            Phone = imported.Phone,
+            Street = imported.Street,
+            PostalCode = imported.PostalCode,
+            City = imported.City,
+        };
+    }
+
+    private static bool IsImportedContact(Contact contact) =>
+        string.Equals(contact.Relation, ImportedContactRelation, StringComparison.OrdinalIgnoreCase);
+
+    private static bool FillIfEmpty(string? existing, string? imported, Action<string> assign)
+    {
+        if (!string.IsNullOrWhiteSpace(existing) || string.IsNullOrWhiteSpace(imported))
+            return false;
+        assign(imported);
+        return true;
+    }
+
+    private static bool NamesEqual(Student existing, ImportedStudent imported) =>
+        string.Equals(existing.FirstName.Trim(), imported.FirstName, StringComparison.OrdinalIgnoreCase)
+        && string.Equals(existing.LastName.Trim(), imported.LastName, StringComparison.OrdinalIgnoreCase);
+
+    private static string ExistingOption(Guid id) => $"{ExistingOptionPrefix}{id:N}";
+    private static string GroupOption(Guid id) => $"{GroupOptionPrefix}{id:N}";
+
+    private static bool TryReadExistingOption(string option, out Guid id)
+    {
+        id = Guid.Empty;
+        return option.StartsWith(ExistingOptionPrefix, StringComparison.Ordinal)
+               && Guid.TryParseExact(option[ExistingOptionPrefix.Length..], "N", out id);
+    }
+
+    private static bool TryReadGroupOption(string option, out Guid id)
+    {
+        id = Guid.Empty;
+        return option.StartsWith(GroupOptionPrefix, StringComparison.Ordinal)
+               && Guid.TryParseExact(option[GroupOptionPrefix.Length..], "N", out id);
+    }
+
+    private static string ImportedDetails(ImportedStudent student) =>
+        $"{student.LastName}, {student.FirstName} · {DateDisplay(student.DateOfBirth)}";
+
+    private static string StudentDetails(Student student) =>
+        $"{student.FullName} · {DateDisplay(student.DateOfBirth)}";
+
+    private static string DateDisplay(DateOnly? date) =>
+        date?.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture) ?? "Geburtsdatum unbekannt";
+
+    private static string Fingerprint(IEnumerable<Student> students)
+    {
+        var builder = new StringBuilder();
+        foreach (var student in students.OrderBy(student => student.Id))
+        {
+            builder.Append(student.Id.ToString("N"))
+                .Append(':').Append(student.UpdatedAt.Ticks)
+                .Append(':').Append(student.FirstName)
+                .Append(':').Append(student.LastName)
+                .Append(':').Append(student.DateOfBirth?.ToString("yyyyMMdd", CultureInfo.InvariantCulture))
+                .Append(':').Append(student.Gender)
+                .Append(':').Append(student.IsActive)
+                .Append(':').Append(string.Join(",", (student.ExternalIds ?? [])
+                    .OrderBy(pair => pair.Key, StringComparer.Ordinal)
+                    .Select(pair => $"{pair.Key}={pair.Value}")))
+                .Append(':').Append(string.Join(",", (student.Contacts ?? [])
+                    .OrderBy(contact => contact.Id)
+                    .Select(contact =>
+                        $"{contact.Id:N}={contact.Name}/{contact.Relation}/{contact.Email}/"
+                        + $"{contact.MobilePhone}/{contact.Phone}/{contact.Street}/"
+                        + $"{contact.PostalCode}/{contact.City}")))
+                .Append('|');
+        }
+        return builder.ToString();
+    }
+
+    private static string GroupStateFingerprint(
+        IEnumerable<LearningGroup> groups,
+        IEnumerable<GroupMembership> memberships,
+        string currentSchoolYear)
+    {
+        var builder = new StringBuilder(currentSchoolYear).Append('|');
+        foreach (var group in groups.OrderBy(group => group.Id))
+            builder.Append(group.Id.ToString("N"))
+                .Append(':').Append(group.UpdatedAt.Ticks)
+                .Append(':').Append(group.Name)
+                .Append(':').Append(group.SchoolYear)
+                .Append(':').Append(group.IsActive)
+                .Append('|');
+        foreach (var membership in memberships.OrderBy(membership => membership.Id))
+            builder.Append(membership.Id.ToString("N"))
+                .Append(':').Append(membership.StudentId.ToString("N"))
+                .Append(':').Append(membership.GroupId.ToString("N"))
+                .Append('|');
+        return builder.ToString();
+    }
+}

LehrerApp.Core/Services/UntisDiffService.cs

@@ -0,0 +1,228 @@
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Services;
+
+public sealed class UntisDiffResult
+{
+    /// SubstitutionEntry-Kandidaten (Id ist immer neu vergeben — der Aufrufer entscheidet über
+    /// ISubstitutionEntryRepository.GetByExternalId, ob ein vorhandener Eintrag aktualisiert
+    /// statt dupliziert werden muss; siehe UntisSyncService).
+    public List<SubstitutionEntry> SubstitutionsToSave { get; init; } = [];
+    public List<UntisSnapshotEntry> SnapshotToSave { get; init; } = [];
+    /// Verschwundene, bereits als Ausfall verarbeitete Zeilen — werden aus dem Snapshot entfernt,
+    /// damit derselbe Ausfall nicht bei jedem weiteren Poll erneut erkannt wird.
+    public List<Guid> SnapshotIdsToDelete { get; init; } = [];
+    /// ExternalIds automatisch erzeugter SubstitutionEntry-Zeilen, die NICHT (mehr) gebraucht
+    /// werden — z.B. weil eine früher erkannte Abweichung sich (durch einen Bugfix oder eine
+    /// erneute Zuordnung) als nicht mehr abweichend herausstellt, oder eine zuvor als fehlend
+    /// gemeldete Stunde jetzt wieder im Feed auftaucht. Nutzer-Feedback: "Es ist immer noch so"
+    /// (nachdem der eigentliche Vergleichsfehler bereits behoben war) — Ursache: einmal erzeugte
+    /// automatische Einträge wurden nie wieder entfernt, selbst wenn der Vergleich sie beim
+    /// nächsten Poll nicht mehr als Abweichung einstufte; sie blieben als Karteileichen stehen.
+    /// Existiert kein Eintrag mit dieser ExternalId, ist das Löschen ein no-op (siehe UntisSyncService).
+    public List<string> SubstitutionExternalIdsToDelete { get; init; } = [];
+}
+
+/// <summary>
+/// Stufe 2 des WebUntis-Abgleichs: ein neuer iCal-Abruf gegen den zuletzt gespeicherten
+/// Schnappschuss (Nutzer-Feedback: "Abgleich mit einem lokalen Backup, um Änderungen zu finden").
+/// Framework-frei wie <see cref="UntisMatchingService"/> — nimmt nur einfache Objekte/Listen
+/// entgegen, kein Datenbankzugriff.
+/// </summary>
+public class UntisDiffService
+{
+    public const int DefaultLookaheadDays = 14;
+
+    public UntisDiffResult Diff(List<UntisIcsEvent> newEvents, List<UntisSnapshotEntry> previousSnapshot,
+        List<UntisSlotMapping> confirmedMappings, DateOnly today, int lookaheadDays = DefaultLookaheadDays,
+        List<SupervisionDuty>? existingSupervisionDuties = null, HashSet<DateOnly>? freeDates = null)
+    {
+        var previousByUid = previousSnapshot.ToDictionary(s => s.Uid);
+        // GroupBy statt direktem ToDictionary: robust gegen mehrere Mappings für denselben Slot
+        // (z.B. Altdaten vor einem Bugfix) - die zuletzt angelegte gewinnt, statt mit einer
+        // ArgumentException abzubrechen und dadurch den gesamten Poll (Timer-Callback) zu killen.
+        var mappingByWeekdayStart = confirmedMappings
+            .Where(m => m.Confirmed && (m.Kind == SubstitutionKind.Lesson ? m.PeriodNumber is not null : m.AfterPeriod is not null))
+            .GroupBy(m => (m.Weekday, m.StartTime))
+            .ToDictionary(g => g.Key, g => g.OrderByDescending(m => m.CreatedAt).First());
+        // Regelmäßige, ohnehin schon jede Woche im Stundenplan sichtbare Aufsichten (siehe
+        // SupervisionDuty/Einstellungen "Aufsichten") - eine bestätigte Supervision-Zuordnung, die
+        // KEINER davon entspricht, ist per Definition eine zusätzliche/Vertretungsaufsicht und soll
+        // deshalb selbst als reguläres Vorkommnis (nicht erst bei Abweichung) im Plan auftauchen.
+        var regularDutyKeys = (existingSupervisionDuties ?? [])
+            .Select(d => (d.Weekday, d.AfterPeriod))
+            .ToHashSet();
+
+        var substitutions = new List<SubstitutionEntry>();
+        var snapshotToSave = new List<UntisSnapshotEntry>();
+        var externalIdsToDelete = new List<string>();
+        var newEventsByUid = newEvents.ToDictionary(e => e.Uid);
+
+        foreach (var evt in newEvents)
+        {
+            snapshotToSave.Add(new UntisSnapshotEntry
+            {
+                Id = previousByUid.TryGetValue(evt.Uid, out var existing) ? existing.Id : Guid.NewGuid(),
+                Uid = evt.Uid, Date = evt.Date, StartTime = evt.StartTime, EndTime = evt.EndTime,
+                Summary = evt.Summary, Location = evt.Location, Description = evt.Description,
+                Status = evt.Status, LastSeenAt = DateTime.UtcNow,
+            });
+
+            if (!mappingByWeekdayStart.TryGetValue((evt.Weekday, evt.StartTime), out var mapping)) continue;
+
+            if (string.Equals(evt.Status, "CANCELLED", StringComparison.OrdinalIgnoreCase))
+            {
+                substitutions.Add(mapping.Kind == SubstitutionKind.Lesson
+                    ? BuildCancelledLesson(evt.Date, mapping, evt.Uid)
+                    : BuildSupervisionNote(evt.Date, mapping, evt.Uid, "Laut WebUntis abgesagt."));
+                continue;
+            }
+
+            if (mapping.Kind == SubstitutionKind.Lesson)
+            {
+                if (DeviationReason(evt, mapping) is { } reason)
+                    substitutions.Add(BuildChangedLesson(evt, mapping, reason));
+                else
+                    externalIdsToDelete.Add(evt.Uid); // war ggf. vorher fälschlich als Vertretung erkannt
+            }
+            else if (!regularDutyKeys.Contains((mapping.Weekday, mapping.AfterPeriod!.Value)))
+            {
+                // Nutzer-Feedback: "Auch die Extra-Aufsicht ist dann nicht im Plan [...] So macht
+                // doch der Sync nur so halb Sinn" - ohne passende reguläre Aufsicht IST dieses
+                // Vorkommnis selbst schon die meldenswerte Vertretung, nicht erst eine Abweichung
+                // davon.
+                substitutions.Add(BuildSupervisionNote(evt.Date, mapping, evt.Uid, "Zusätzliche Aufsicht laut WebUntis."));
+            }
+            else
+            {
+                externalIdsToDelete.Add(evt.Uid); // jetzt als reguläre Aufsicht erkannt, keine Meldung mehr nötig
+            }
+        }
+
+        // Aufsichten: weiterhin rein reaktiv gegen den letzten Snapshot (kein fester wöchentlicher
+        // Anspruch, siehe "Zusätzliche Aufsicht" oben - ein Verschwinden ist nur meldenswert, wenn
+        // die Aufsicht vorher tatsächlich einmal gesehen wurde).
+        var snapshotIdsToDelete = new List<Guid>();
+        foreach (var previous in previousSnapshot)
+        {
+            if (newEventsByUid.ContainsKey(previous.Uid)) continue;
+            if (previous.Date < today || previous.Date > today.AddDays(lookaheadDays)) continue;
+            if (!mappingByWeekdayStart.TryGetValue((previous.Date.DayOfWeek, previous.StartTime), out var mapping)) continue;
+            if (mapping.Kind != SubstitutionKind.Supervision) continue;
+
+            var vanishedId = BuildVanishedExternalId(previous.Date, mapping);
+            substitutions.Add(BuildSupervisionNote(previous.Date, mapping, vanishedId, "Laut WebUntis entfallen oder übernommen."));
+            snapshotIdsToDelete.Add(previous.Id);
+        }
+
+        // Unterrichtsstunden: zusätzlich AKTIV prüfen, ob für jedes vom Feed bereits abgedeckte
+        // künftige Datum dieses Wochentags ein passender Termin existiert - rein reaktives
+        // Schnappschuss-Diffing (oben) erkennt nur Termine, die zwischen zwei Abrufen
+        // VERSCHWINDEN, nicht solche, die schon beim allerersten Abruf nie im Feed auftauchten
+        // (Nutzer-Feedback: "Am 27.08. fällt eine Stunde NAT in der 8c aus. Dieser Ausfall steht
+        // nicht im Plan [...] die Klasse ist dort weg, der Unterricht wird definitiv nicht
+        // stattfinden" - WebUntis hatte diesen Ausfall von Anfang an nie als Termin gelistet, es
+        // gab also nie einen Schnappschuss-Eintrag, der hätte "verschwinden" können). Begrenzt auf
+        // das vom Feed tatsächlich abgedeckte Zeitfenster (jüngstes gesehenes Datum), damit nicht
+        // Tage jenseits des vom Feed veröffentlichten Horizonts fälschlich als Ausfall gelten, und
+        // um Ferien/Feiertage bereinigt (freeDates), an denen WebUntis ohnehin keinen Termin führt.
+        if (newEvents.Count > 0)
+        {
+            var maxKnownDate = newEvents.Max(e => e.Date);
+            var horizonEnd = maxKnownDate < today.AddDays(lookaheadDays) ? maxKnownDate : today.AddDays(lookaheadDays);
+            var eventDates = newEvents.Select(e => (e.Date, e.StartTime)).ToHashSet();
+            var freeDateSet = freeDates ?? [];
+
+            foreach (var mapping in mappingByWeekdayStart.Values.Where(m => m.Kind == SubstitutionKind.Lesson))
+            {
+                for (var date = today; date <= horizonEnd; date = date.AddDays(1))
+                {
+                    if (date.DayOfWeek != mapping.Weekday) continue;
+                    if (freeDateSet.Contains(date)) continue;
+                    if (eventDates.Contains((date, mapping.StartTime)))
+                    {
+                        // War evtl. an einem früheren Poll (noch) als fehlend gemeldet - jetzt
+                        // wieder im Feed vorhanden, die alte Meldung räumt sich damit selbst ab.
+                        externalIdsToDelete.Add(BuildMissingExternalId(date, mapping));
+                        continue;
+                    }
+
+                    substitutions.Add(BuildCancelledLesson(date, mapping, BuildMissingExternalId(date, mapping)));
+                }
+            }
+        }
+
+        return new UntisDiffResult
+        {
+            SubstitutionsToSave = substitutions, SnapshotToSave = snapshotToSave,
+            SnapshotIdsToDelete = snapshotIdsToDelete, SubstitutionExternalIdsToDelete = externalIdsToDelete,
+        };
+    }
+
+    // Nutzer-Feedback: "Ich habe das aufgelöst und den Mathematik E-Kurs ausgewählt [...] Diese
+    // manuelle Verknüpfung ist aber jetzt scheinbar vergessen" — Ursache: ClassToken wird beim
+    // Bestätigen kompakt ohne Leerzeichen gespeichert ("10a;10b;10c"), WebUntis trennt die
+    // Klassen in DESCRIPTION aber mit "; " (Semikolon + Leerzeichen), z.B. "10a; 10b; 10c;
+    // Gastro HED". Ein erster Fix entfernte Leerzeichen nur auf der evt.Description-Seite — blieb
+    // aber weiterhin falsch, wenn mapping.ClassToken selbst ein eingebettetes Leerzeichen enthält
+    // (z.B. wenn die reale DESCRIPTION mehrere Klassen NICHT mit ";", sondern mit "," trennt:
+    // ExtractClassTokens fand dann kein ";" und behandelte "10a, 10b, 10c" als EIN Token mit
+    // Leerzeichen/Kommas statt drei getrennte - ein Vergleich gegen die leerzeichenbereinigte
+    // Description konnte dieses Token dann NIE mehr finden, unabhängig von der Auswahl im Dialog).
+    // Fix: beide Seiten symmetrisch von Leerzeichen befreien (gleiches Prinzip wie
+    // UntisMatchingService.Normalize für den Gruppennamen-Abgleich), plus ExtractClassTokens
+    // akzeptiert jetzt sowohl ";" als auch "," als Trennzeichen zwischen Klassen.
+    //
+    // Trotzdem meldet der Nutzer weiterhin falsche Vertretungen für genau diese Art Muster
+    // ("Immer die Kurse mit Lerngruppen, die aus mehreren Klassen zusammengesetzt sind") - der
+    // genaue Grund lässt sich ohne Einblick in die echten DESCRIPTION-Werte aus seinem Feed nicht
+    // mehr blind erraten. Statt eines weiteren ungetesteten Rateversuchs schreibt eine abweichende
+    // Vertretung deshalb jetzt den GENAUEN Vergleich (roher evt.Summary/evt.Description gegen den
+    // gespeicherten mapping.Summary/ClassToken) in ihre eigene Beschreibung - der Nutzer kann die
+    // Ursache dann direkt im Stundenplan ablesen, ohne die iCal-URL erneut teilen zu müssen.
+    private static string? DeviationReason(UntisIcsEvent evt, UntisSlotMapping mapping)
+    {
+        if (evt.Summary != mapping.Summary)
+            return $"Fach weicht ab: WebUntis meldet „{evt.Summary ?? "(leer)"}“, erwartet war „{mapping.Summary ?? "(leer)"}“.";
+        if (!RemoveWhitespace(evt.Description).Contains(RemoveWhitespace(mapping.ClassToken)))
+            return $"Klasse weicht ab: WebUntis-Beschreibung „{evt.Description}“ enthält nicht den erwarteten Klassen-Token „{mapping.ClassToken}“.";
+        return null;
+    }
+
+    private static string RemoveWhitespace(string value) =>
+        new(value.Where(c => !char.IsWhiteSpace(c)).ToArray());
+
+    private static SubstitutionEntry BuildCancelledLesson(DateOnly date, UntisSlotMapping mapping, string externalId) => new()
+    {
+        Date = date, Kind = SubstitutionKind.Cancelled, PeriodNumber = mapping.PeriodNumber,
+        Description = "Automatisch über WebUntis-Abgleich erkannt.", ExternalId = externalId,
+    };
+
+    private static SubstitutionEntry BuildChangedLesson(UntisIcsEvent evt, UntisSlotMapping mapping, string reason) => new()
+    {
+        Date = evt.Date, Kind = SubstitutionKind.Lesson, PeriodNumber = mapping.PeriodNumber,
+        GroupId = mapping.GroupId, GroupLabel = mapping.ClassToken,
+        Description = $"WebUntis: {evt.Summary ?? "?"} · {evt.Location} (regulär: {mapping.Summary ?? "?"}) — {reason}",
+        ExternalId = evt.Uid,
+    };
+
+    // Aufsichten haben keinen TimetableSlot im Hintergrund (siehe SubstitutionEntry.Kind-Doc) -
+    // deshalb Kind=Supervision statt Cancelled, auch wenn der Termin verschwunden ist.
+    private static SubstitutionEntry BuildSupervisionNote(DateOnly date, UntisSlotMapping mapping,
+        string externalId, string reason) => new()
+    {
+        Date = date, Kind = SubstitutionKind.Supervision, AfterPeriod = mapping.AfterPeriod,
+        Description = $"Automatisch über WebUntis-Abgleich erkannt: {reason}", ExternalId = externalId,
+    };
+
+    // Ein verschwundener Termin hat keine eigene UID mehr im neuen Fetch - ein aus Slot+Datum
+    // abgeleiteter Ersatzschlüssel dient weiter als Idempotenz-Schlüssel für spätere Polls.
+    private static string BuildVanishedExternalId(DateOnly date, UntisSlotMapping mapping) =>
+        $"vanished-{mapping.Id}-{date:yyyyMMdd}";
+
+    // Für eine von Anfang an fehlende Stunde gibt es nie eine echte iCal-UID - ein aus
+    // Slot+Datum abgeleiteter Schlüssel macht wiederholte Polls trotzdem idempotent (Update
+    // statt Duplikat), unabhängig vom Snapshot-Stand.
+    private static string BuildMissingExternalId(DateOnly date, UntisSlotMapping mapping) =>
+        $"missing-{mapping.Id}-{date:yyyyMMdd}";
+}

LehrerApp.Core/Services/UntisMatchingService.cs

@@ -0,0 +1,241 @@
+using LehrerApp.Core.Models;
+
+namespace LehrerApp.Core.Services;
+
+/// <summary>Ein aus vielen Wochen-Vorkommen abgeleitetes reguläres WebUntis-Wochenmuster
+/// (Nutzer-Feedback: "eine fuzzy logic, die erst mal die regulären Fächer meiner ical meinem
+/// Stundenplan in der App zuordnet").</summary>
+public sealed class UntisWeeklyPattern
+{
+    public DayOfWeek Weekday { get; init; }
+    public TimeOnly StartTime { get; init; }
+    public TimeOnly EndTime { get; init; }
+    public string? Summary { get; init; }
+    /// Klassen-Token(s) aus DESCRIPTION, Lehrkraft-Kürzel bereits abgetrennt — bei kombinierten
+    /// Gruppen mehrere Einträge (z.B. ["10a", "10b", "10c", "Gastro"]).
+    public List<string> ClassTokens { get; init; } = [];
+    public int OccurrenceCount { get; init; }
+}
+
+public sealed class UntisSlotMatch
+{
+    public required UntisWeeklyPattern Pattern { get; init; }
+    /// Erste/primäre Stunde, gesetzt wenn das Muster einen Klassenbezug hat (Unterrichtsstunde) —
+    /// entspricht CoveredPeriods[0].
+    public int? PeriodNumber { get; init; }
+    /// ALLE Stunden, die dieser WebUntis-Termin überdeckt — bei einer Doppelstunde meldet WebUntis
+    /// EINEN Termin über beide Stundenzeiten hinweg (z.B. 07:50–09:20 für Stunde 1+2), während der
+    /// Stundenplan der App dafür ZWEI TimetableSlot-Einträge (Stunde 1 UND Stunde 2) haben kann.
+    /// Ohne diese Liste würde die zweite Stunde nie als "zugeordnet" gelten, selbst nachdem der
+    /// Nutzer das Muster bestätigt hat (Nutzer-Feedback: "ich habe aber jetzt alles zugeordnet,
+    /// und trotzdem erhalte ich die Warnung, dass 16 Stunden ohne Untis-Zuordnung sind").
+    public List<int> CoveredPeriods { get; init; } = [];
+    /// Gesetzt, wenn das Muster KEINEN Klassenbezug hat (z.B. Aufsicht/Springstunde) — die Pause
+    /// direkt vor der Startzeit (0 = vor der 1. Stunde), siehe UntisSlotMapping.AfterPeriod.
+    public int? AfterPeriod { get; init; }
+    public bool IsSupervisionCandidate => Pattern.ClassTokens.Count == 0;
+    public Guid? SuggestedGroupId { get; init; }
+    public bool IsConfident { get; init; }
+}
+
+public sealed class UntisMatchResult
+{
+    public List<UntisSlotMatch> Matches { get; init; } = [];
+    /// Vorhandene TimetableSlots, zu denen sich kein passendes WebUntis-Wochenmuster finden ließ
+    /// (Nutzer-Feedback: "oder der Stundenplan gar nicht mehr passt").
+    public List<TimetableSlot> UnmatchedTimetableSlots { get; init; } = [];
+}
+
+/// <summary>
+/// Stufe 1 des WebUntis-Abgleichs (siehe TODO.md): aus einem vollen iCal-Abruf (typischerweise
+/// ein Schuljahr an Einzelterminen, siehe IcsParser) das reguläre Wochenmuster ableiten und gegen
+/// den in der App gepflegten Stundenplan (TimetableSlot/LearningGroup) abgleichen. Framework-frei
+/// wie AttendanceBalanceService — reine Objekte rein, reine Objekte raus, ohne Datenbank- oder
+/// HTTP-Zugriff, dadurch ohne Mocking testbar.
+/// </summary>
+public class UntisMatchingService
+{
+    // Kleine Toleranz gegen Minuten-Rundungsdifferenzen zwischen dem konfigurierten Stundenraster
+    // und den tatsächlichen WebUntis-Zeiten.
+    private const int StartTimeToleranceMinutes = 3;
+
+    public UntisMatchResult BuildMatches(List<UntisIcsEvent> events, List<LearningGroup> groups,
+        List<TimetableSlot> timetableSlots, PeriodScheduleService periodSchedule)
+    {
+        var teacherToken = DetectTeacherToken(events);
+        var patterns = BuildWeeklyPatterns(events, teacherToken);
+
+        var matches = new List<UntisSlotMatch>();
+        foreach (var pattern in patterns)
+        {
+            // Kein Klassenbezug (z.B. Aufsicht/Springstunde, Nutzer-Feedback: "Zwei Termine sind
+            // meine Aufsichten, die nicht zugeordnet werden können") - liegt typischerweise in
+            // einer Pause, nicht auf einer Unterrichtsstunden-Startzeit, deshalb eigener,
+            // grundsätzlich immer auflösbarer Weg statt ResolvePeriodNumber.
+            if (pattern.ClassTokens.Count == 0)
+            {
+                matches.Add(new UntisSlotMatch { Pattern = pattern, AfterPeriod = ResolveAfterPeriod(pattern.StartTime, periodSchedule) });
+                continue;
+            }
+
+            var coveredPeriods = ResolveCoveredPeriods(pattern.StartTime, pattern.EndTime, periodSchedule);
+            var periodNumber = coveredPeriods.Count > 0 ? coveredPeriods[0] : ResolvePeriodNumber(pattern.StartTime, periodSchedule);
+            var (groupId, confident) = ResolveGroup(pattern.ClassTokens, periodNumber, groups, timetableSlots);
+            matches.Add(new UntisSlotMatch
+            {
+                Pattern = pattern, PeriodNumber = periodNumber, CoveredPeriods = coveredPeriods,
+                SuggestedGroupId = groupId, IsConfident = confident,
+            });
+        }
+
+        var matchedSlotKeys = matches
+            .Where(m => m.SuggestedGroupId is not null)
+            .SelectMany(m => (m.CoveredPeriods.Count > 0 ? m.CoveredPeriods : m.PeriodNumber is { } p ? [p] : [])
+                .Select(period => (m.Pattern.Weekday, period, m.SuggestedGroupId!.Value)))
+            .ToHashSet();
+        var unmatchedSlots = timetableSlots
+            .Where(s => !matchedSlotKeys.Contains((s.Weekday, s.PeriodNumber, s.GroupId)))
+            .ToList();
+
+        return new UntisMatchResult { Matches = matches, UnmatchedTimetableSlots = unmatchedSlots };
+    }
+
+    /// Häufigstes letztes Wort in DESCRIPTION über alle Termine — bei einem persönlichen Feed
+    /// konstant das eigene Kürzel, bewusst nicht hartkodiert (siehe Planungsdokument).
+    private static string? DetectTeacherToken(List<UntisIcsEvent> events) =>
+        events
+            .Select(e => LastToken(e.Description))
+            .Where(t => t is not null)
+            .GroupBy(t => t)
+            .OrderByDescending(g => g.Count())
+            .Select(g => g.Key)
+            .FirstOrDefault();
+
+    private static string? LastToken(string description)
+    {
+        var parts = description.Split(' ', StringSplitOptions.RemoveEmptyEntries);
+        return parts.Length == 0 ? null : parts[^1];
+    }
+
+    private static List<UntisWeeklyPattern> BuildWeeklyPatterns(List<UntisIcsEvent> events, string? teacherToken)
+    {
+        var withTokens = events.Select(e => new
+        {
+            Event = e,
+            ClassTokens = ExtractClassTokens(e.Description, teacherToken),
+        });
+
+        var patterns = new List<UntisWeeklyPattern>();
+        foreach (var group in withTokens.GroupBy(x => (x.Event.Weekday, x.Event.StartTime, x.Event.EndTime)))
+        {
+            var modal = group
+                .GroupBy(x => (x.Event.Summary, ClassTokens: string.Join(";", x.ClassTokens)))
+                .OrderByDescending(g => g.Count())
+                .First();
+
+            patterns.Add(new UntisWeeklyPattern
+            {
+                Weekday = group.Key.Weekday,
+                StartTime = group.Key.StartTime,
+                EndTime = group.Key.EndTime,
+                Summary = modal.Key.Summary,
+                ClassTokens = modal.Key.ClassTokens.Length == 0
+                    ? [] : modal.Key.ClassTokens.Split(';').ToList(),
+                OccurrenceCount = modal.Count(),
+            });
+        }
+        return patterns;
+    }
+
+    // "10a; 10b; 10c; Gastro HED" -> ["10a", "10b", "10c", "Gastro"] (Lehrkraft-Token entfernt).
+    // Trennzeichen zwischen mehreren Klassen wurde nur an einem einzelnen echten Beispiel mit EINER
+    // Klasse verifiziert (siehe Planungsdokument) - für kombinierte/differenzierte Gruppen ist nicht
+    // sicher belegt, ob WebUntis ";" oder "," verwendet, deshalb werden beide akzeptiert statt sich
+    // auf eine Annahme festzulegen.
+    private static readonly char[] ClassTokenSeparators = [';', ','];
+
+    private static List<string> ExtractClassTokens(string description, string? teacherToken)
+    {
+        var withoutTeacher = teacherToken is not null && description.EndsWith(teacherToken)
+            ? description[..^teacherToken.Length].TrimEnd()
+            : description;
+        if (string.IsNullOrWhiteSpace(withoutTeacher)) return [];
+        return withoutTeacher.Split(ClassTokenSeparators, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries).ToList();
+    }
+
+    private static int? ResolvePeriodNumber(TimeOnly startTime, PeriodScheduleService periodSchedule)
+    {
+        var exact = periodSchedule.Periods.FirstOrDefault(p => p.Start == startTime);
+        if (exact is not null) return exact.PeriodNumber;
+
+        var closest = periodSchedule.Periods
+            .Select(p => (Period: p, DiffMinutes: Math.Abs((p.Start.ToTimeSpan() - startTime.ToTimeSpan()).TotalMinutes)))
+            .Where(t => t.DiffMinutes <= StartTimeToleranceMinutes)
+            .OrderBy(t => t.DiffMinutes)
+            .FirstOrDefault();
+        return closest.Period?.PeriodNumber;
+    }
+
+    // Alle konfigurierten Stunden, die vollständig innerhalb [start, end) liegen — bei einer
+    // einzelnen Stunde genau eine, bei einer von WebUntis zu einem Termin zusammengefassten
+    // Doppelstunde entsprechend zwei (siehe UntisSlotMatch.CoveredPeriods).
+    private static List<int> ResolveCoveredPeriods(TimeOnly start, TimeOnly end, PeriodScheduleService periodSchedule)
+    {
+        var tolerance = TimeSpan.FromMinutes(StartTimeToleranceMinutes);
+        return periodSchedule.Periods
+            .Where(p => p.Start.ToTimeSpan() >= start.ToTimeSpan() - tolerance
+                     && p.End.ToTimeSpan() <= end.ToTimeSpan() + tolerance)
+            .OrderBy(p => p.PeriodNumber)
+            .Select(p => p.PeriodNumber)
+            .ToList();
+    }
+
+    // Die Pause direkt vor startTime: die zuletzt endende konfigurierte Stunde davor, 0 (vor der
+    // 1. Stunde) falls keine liegt - anders als ResolvePeriodNumber immer auflösbar, da eine
+    // Pause per Definition zwischen/vor Stunden liegt statt exakt auf einer Startzeit.
+    private static int ResolveAfterPeriod(TimeOnly startTime, PeriodScheduleService periodSchedule) =>
+        periodSchedule.Periods
+            .Where(p => p.End <= startTime)
+            .OrderByDescending(p => p.End)
+            .Select(p => (int?)p.PeriodNumber)
+            .FirstOrDefault() ?? 0;
+
+    private static (Guid? GroupId, bool Confident) ResolveGroup(List<string> classTokens, int? periodNumber,
+        List<LearningGroup> groups, List<TimetableSlot> timetableSlots)
+    {
+        if (classTokens.Count == 0) return (null, false);
+
+        var candidates = classTokens
+            .Select(token => ResolveSingleGroup(token, groups))
+            .Where(g => g is not null)
+            .Select(g => g!)
+            .Distinct()
+            .ToList();
+
+        if (candidates.Count == 1) return (candidates[0].Id, true);
+        if (candidates.Count == 0) return (null, false);
+
+        // Mehrere Klassen im selben Termin (kombinierte/differenzierte Gruppen) — die Gruppe
+        // bevorzugen, die an dieser Stelle bereits einen TimetableSlot hat.
+        if (periodNumber is { } period)
+        {
+            var withExistingSlot = candidates
+                .Where(g => timetableSlots.Any(s => s.GroupId == g.Id && s.PeriodNumber == period))
+                .ToList();
+            if (withExistingSlot.Count == 1) return (withExistingSlot[0].Id, true);
+        }
+        return (null, false); // mehrdeutig - der Nutzer entscheidet im Review-Dialog
+    }
+
+    private static LearningGroup? ResolveSingleGroup(string token, List<LearningGroup> groups)
+    {
+        var exact = groups.FirstOrDefault(g => string.Equals(g.Name, token, StringComparison.OrdinalIgnoreCase));
+        if (exact is not null) return exact;
+
+        var normalizedToken = Normalize(token);
+        return groups.FirstOrDefault(g => Normalize(g.Name) == normalizedToken);
+    }
+
+    private static string Normalize(string value) =>
+        new(value.Where(c => !char.IsWhiteSpace(c)).Select(char.ToLowerInvariant).ToArray());
+}

LehrerApp.Data.Tests/ChangeHookCascadeTests.cs

@@ -0,0 +1,196 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Data.Repositories;
+using Xunit;
+
+namespace LehrerApp.Data.Tests;
+
+// Baustein 4: pro öffentlichem Repository-Aufruf genau EIN Sync-Ereignis, auch wenn die
+// eigentliche Kaskade mehrere Collections betrifft. Batch-Methoden feuern ein Ereignis pro
+// betroffener Entität.
+public sealed class ChangeHookCascadeTests
+{
+    private static LiteDbContext NewInMemoryContext() => new(new MemoryStream());
+
+    [Fact]
+    public void GroupRepository_Delete_LoestGenauEinEreignisAusTrotzKaskade()
+    {
+        using var db = NewInMemoryContext();
+        var groupRepo = new GroupRepository(db);
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        groupRepo.Save(group);
+        db.Grades.Insert(new Grade { GroupId = group.Id, StudentId = Guid.NewGuid() });
+        db.Exams.Insert(new Exam { GroupId = group.Id });
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        groupRepo.Delete(group.Id);
+
+        var call = Assert.Single(calls);
+        Assert.Equal((nameof(LearningGroup), "Delete"), call);
+    }
+
+    [Fact]
+    public void ExamRepository_Delete_LoestGenauEinEreignisAusTrotzErgebnisKaskade()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new ExamRepository(db);
+        var exam = new Exam { GroupId = Guid.NewGuid() };
+        repo.Save(exam);
+        db.ExamResults.Insert(new ExamResult { ExamId = exam.Id, StudentId = Guid.NewGuid() });
+        db.ExamResults.Insert(new ExamResult { ExamId = exam.Id, StudentId = Guid.NewGuid() });
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        repo.Delete(exam.Id);
+
+        var call = Assert.Single(calls);
+        Assert.Equal((nameof(Exam), "Delete"), call);
+    }
+
+    [Fact]
+    public void ParticipationSessionRepository_Delete_LoestGenauEinEreignisAusTrotzEintragKaskade()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new ParticipationSessionRepository(db);
+        var session = new ParticipationSession { GroupId = Guid.NewGuid() };
+        repo.Save(session);
+        db.ParticipationEntries.Insert(new ParticipationEntry { SessionId = session.Id, StudentId = Guid.NewGuid() });
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        repo.Delete(session.Id);
+
+        var call = Assert.Single(calls);
+        Assert.Equal((nameof(ParticipationSession), "Delete"), call);
+    }
+
+    [Fact]
+    public void ExamResultRepository_SaveMany_LoestEinEreignisProEintragAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new ExamResultRepository(db);
+        var results = new List<ExamResult>
+        {
+            new() { ExamId = Guid.NewGuid(), StudentId = Guid.NewGuid() },
+            new() { ExamId = Guid.NewGuid(), StudentId = Guid.NewGuid() },
+            new() { ExamId = Guid.NewGuid(), StudentId = Guid.NewGuid() },
+        };
+        var calls = new List<(string EntityType, string EntityId, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, id, op));
+
+        repo.SaveMany(results);
+
+        Assert.Equal(3, calls.Count);
+        Assert.All(calls, c => Assert.Equal((nameof(ExamResult), "Save"), (c.EntityType, c.Operation)));
+        Assert.Equal(results.Select(r => r.Id.ToString()).OrderBy(x => x), calls.Select(c => c.EntityId).OrderBy(x => x));
+    }
+
+    [Fact]
+    public void ParticipationRepository_SaveMany_LoestEinEreignisProEintragAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new ParticipationRepository(db);
+        var sessionId = Guid.NewGuid();
+        var entries = new List<ParticipationEntry>
+        {
+            new() { SessionId = sessionId, StudentId = Guid.NewGuid() },
+            new() { SessionId = sessionId, StudentId = Guid.NewGuid() },
+        };
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        repo.SaveMany(entries);
+
+        Assert.Equal(2, calls.Count);
+        Assert.All(calls, c => Assert.Equal((nameof(ParticipationEntry), "Save"), c));
+    }
+
+    [Fact]
+    public void ParticipationRepository_DeleteBySession_LoestEinEreignisProEintragAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new ParticipationRepository(db);
+        var sessionId = Guid.NewGuid();
+        repo.Save(new ParticipationEntry { SessionId = sessionId, StudentId = Guid.NewGuid() });
+        repo.Save(new ParticipationEntry { SessionId = sessionId, StudentId = Guid.NewGuid() });
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        repo.DeleteBySession(sessionId);
+
+        Assert.Equal(2, calls.Count);
+        Assert.All(calls, c => Assert.Equal((nameof(ParticipationEntry), "Delete"), c));
+    }
+
+    [Fact]
+    public void CompetencyDomainRepository_ReplaceForSubjectAndGrade_LoestDeleteFuerAlteUndSaveFuerNeueAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new CompetencyDomainRepository(db);
+        var subjectId = Guid.NewGuid();
+        repo.Save(new CompetencyDomain { SubjectId = subjectId, GradeLevel = 8, Name = "Alt" });
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        repo.ReplaceForSubjectAndGrade(subjectId, 8,
+        [
+            new CompetencyDomain { Name = "Neu 1" },
+            new CompetencyDomain { Name = "Neu 2" },
+        ]);
+
+        Assert.Equal(3, calls.Count);
+        Assert.Equal(1, calls.Count(c => c.Operation == "Delete"));
+        Assert.Equal(2, calls.Count(c => c.Operation == "Save"));
+        Assert.All(calls, c => Assert.Equal(nameof(CompetencyDomain), c.EntityType));
+    }
+
+    [Fact]
+    public void CompetencyDomainRepository_DeleteBySubjectAndGrade_LoestEinEreignisProDomaenAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new CompetencyDomainRepository(db);
+        var subjectId = Guid.NewGuid();
+        repo.Save(new CompetencyDomain { SubjectId = subjectId, GradeLevel = 8, Name = "A" });
+        repo.Save(new CompetencyDomain { SubjectId = subjectId, GradeLevel = 8, Name = "B" });
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        repo.DeleteBySubjectAndGrade(subjectId, 8);
+
+        Assert.Equal(2, calls.Count);
+        Assert.All(calls, c => Assert.Equal((nameof(CompetencyDomain), "Delete"), c));
+    }
+
+    [Fact]
+    public void DocumentationRepository_Delete_LoestSaveMitIsDeletedAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new DocumentationRepository(db);
+        var doc = new Documentation { StudentId = Guid.NewGuid(), Title = "Gespräch" };
+        repo.Save(doc);
+        var calls = new List<(string EntityType, string Operation, object? Payload)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op, payload));
+
+        repo.Delete(doc.Id);
+
+        var call = Assert.Single(calls);
+        Assert.Equal((nameof(Documentation), "Save"), (call.EntityType, call.Operation));
+        Assert.True(((Documentation)call.Payload!).IsDeleted);
+    }
+
+    [Fact]
+    public void DocumentationRepository_HardDelete_LoestDeleteAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new DocumentationRepository(db);
+        var doc = new Documentation { StudentId = Guid.NewGuid(), Title = "Gespräch" };
+        repo.Save(doc);
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        repo.HardDelete(doc.Id);
+
+        var call = Assert.Single(calls);
+        Assert.Equal((nameof(Documentation), "Delete"), call);
+    }
+}

LehrerApp.Data.Tests/ChangeHookMatrixTests.cs

@@ -0,0 +1,204 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Data.Repositories;
+using Xunit;
+
+namespace LehrerApp.Data.Tests;
+
+// Baustein 3: prüft für jedes der ~19 "einfachen" Repositories (keine Kaskaden/Batches), dass
+// Save/Delete den OnChange-Hook mit dem korrekten EntityType auslöst. Tabellengetrieben statt
+// eine Datei pro Repository, um die immer gleiche Prüfung nicht 19x zu wiederholen.
+public sealed class ChangeHookMatrixTests
+{
+    public static IEnumerable<object[]> Cases()
+    {
+        yield return Case("SeatingPlan", nameof(SeatingPlan), db =>
+        {
+            var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+            new GroupRepository(db).Save(group);
+            var repo = new SeatingPlanRepository(db);
+            var plan = new SeatingPlan { GroupId = group.Id, Name = "Standard", Rows = 2, Columns = 2 };
+            repo.Save(plan);
+            return (plan.Id, () => repo.Delete(plan.Id));
+        });
+
+        yield return Case("GroupMembership", nameof(GroupMembership), db =>
+        {
+            var repo = new GroupMembershipRepository(db);
+            var membership = new GroupMembership { StudentId = Guid.NewGuid(), GroupId = Guid.NewGuid() };
+            repo.Save(membership);
+            return (membership.Id, () => repo.Delete(membership.Id));
+        });
+
+        yield return Case("GradingKeyTemplate", nameof(GradingKeyTemplate), db =>
+        {
+            var repo = new GradingKeyTemplateRepository(db);
+            var template = new GradingKeyTemplate { Name = "Standard", GradingSystem = GradingSystem.Points0To15 };
+            repo.Save(template);
+            return (template.Id, () => repo.Delete(template.Id));
+        });
+
+        yield return Case("Grade", nameof(Grade), db =>
+        {
+            var repo = new GradeRepository(db);
+            var grade = new Grade { GroupId = Guid.NewGuid(), StudentId = Guid.NewGuid(), Value = "2" };
+            repo.Save(grade);
+            return (grade.Id, () => repo.Delete(grade.Id));
+        });
+
+        yield return Case("GradingScheme", nameof(GradingScheme), db =>
+        {
+            var repo = new GradingSchemeRepository(db);
+            var scheme = new GradingScheme { GroupId = Guid.NewGuid(), ExamsPercent = 60, ParticipationPercent = 30, OtherPercent = 10 };
+            repo.Save(scheme);
+            return (scheme.Id, () => repo.Delete(scheme.Id));
+        });
+
+        yield return Case("ReportGrade", nameof(ReportGrade), db =>
+        {
+            var repo = new ReportGradeRepository(db);
+            var grade = new ReportGrade { StudentId = Guid.NewGuid(), GroupId = Guid.NewGuid(), Period = "1. Halbjahr" };
+            repo.Save(grade);
+            return (grade.Id, () => repo.Delete(grade.Id));
+        });
+
+        yield return Case("Unit", nameof(Unit), db =>
+        {
+            var repo = new UnitRepository(db);
+            var unit = new Unit { GroupId = Guid.NewGuid(), Title = "Einheit" };
+            repo.Save(unit);
+            return (unit.Id, () => repo.Delete(unit.Id));
+        });
+
+        yield return Case("Lesson", nameof(Lesson), db =>
+        {
+            var repo = new LessonRepository(db);
+            var lesson = new Lesson { UnitId = Guid.NewGuid(), GroupId = Guid.NewGuid(), Date = new DateOnly(2026, 3, 12) };
+            repo.Save(lesson);
+            return (lesson.Id, () => repo.Delete(lesson.Id));
+        });
+
+        yield return Case("WorkTask", nameof(WorkTask), db =>
+        {
+            var repo = new WorkTaskRepository(db);
+            var task = new WorkTask { Title = "Klausur korrigieren", Category = TaskCategory.Correction };
+            repo.Save(task);
+            return (task.Id, () => repo.Delete(task.Id));
+        });
+
+        yield return Case("TimeEntry", nameof(TimeEntry), db =>
+        {
+            var repo = new TimeEntryRepository(db);
+            var entry = new TimeEntry { Date = new DateOnly(2026, 3, 12), DurationMinutes = 30, Category = "Vorbereitung" };
+            repo.Save(entry);
+            return (entry.Id, () => repo.Delete(entry.Id));
+        });
+
+        yield return Case("ParticipationAspect", nameof(ParticipationAspect), db =>
+        {
+            var repo = new ParticipationAspectRepository(db);
+            var aspect = new ParticipationAspect { GroupId = Guid.NewGuid(), Key = "quality", Label = "Qualität" };
+            repo.Save(aspect);
+            return (aspect.Id, () => repo.Delete(aspect.Id));
+        });
+
+        yield return Case("ParticipationSection", nameof(ParticipationSection), db =>
+        {
+            var repo = new ParticipationSectionRepository(db);
+            var section = new ParticipationSection
+            {
+                GroupId = Guid.NewGuid(), Label = "1. Abschnitt",
+                StartDate = new DateOnly(2026, 3, 1), EndDate = new DateOnly(2026, 4, 1),
+            };
+            repo.Save(section);
+            return (section.Id, () => repo.Delete(section.Id));
+        });
+
+        yield return Case("Subject", nameof(Subject), db =>
+        {
+            var repo = new SubjectRepository(db);
+            var subject = new Subject { Name = "Chemie" };
+            repo.Save(subject);
+            return (subject.Id, () => repo.Delete(subject.Id));
+        });
+
+        yield return Case("ShorthandCode", nameof(ShorthandCode), db =>
+        {
+            var repo = new ShorthandCodeRepository(db);
+            var code = new ShorthandCode { Code = "Tb", Label = "Tafelbild" };
+            repo.Save(code);
+            return (code.Id, () => repo.Delete(code.Id));
+        });
+
+        yield return Case("AlternativeLessonPath", nameof(AlternativeLessonPath), db =>
+        {
+            var repo = new AlternativeLessonPathRepository(db);
+            var path = new AlternativeLessonPath { Name = "Kurzversion" };
+            repo.Save(path);
+            return (path.Id, () => repo.Delete(path.Id));
+        });
+
+        yield return Case("TimetableSlot", nameof(TimetableSlot), db =>
+        {
+            var repo = new TimetableSlotRepository(db);
+            var slot = new TimetableSlot { GroupId = Guid.NewGuid(), Weekday = DayOfWeek.Tuesday, PeriodNumber = 3 };
+            repo.Save(slot);
+            return (slot.Id, () => repo.Delete(slot.Id));
+        });
+
+        yield return Case("SchoolHoliday", nameof(SchoolHoliday), db =>
+        {
+            var repo = new SchoolHolidayRepository(db);
+            var holiday = new SchoolHoliday { Name = "Osterferien", StartDate = new DateOnly(2026, 3, 30), EndDate = new DateOnly(2026, 4, 10) };
+            repo.Save(holiday);
+            return (holiday.Id, () => repo.Delete(holiday.Id));
+        });
+
+        yield return Case("SupervisionDuty", nameof(SupervisionDuty), db =>
+        {
+            var repo = new SupervisionDutyRepository(db);
+            var duty = new SupervisionDuty { Weekday = DayOfWeek.Monday, AfterPeriod = 2, Location = "Pausenhof" };
+            repo.Save(duty);
+            return (duty.Id, () => repo.Delete(duty.Id));
+        });
+
+        yield return Case("SubstitutionEntry", nameof(SubstitutionEntry), db =>
+        {
+            var repo = new SubstitutionEntryRepository(db);
+            var entry = new SubstitutionEntry { Date = new DateOnly(2026, 3, 12), Kind = SubstitutionKind.Lesson, PeriodNumber = 3, Description = "Vertretung 8a" };
+            repo.Save(entry);
+            return (entry.Id, () => repo.Delete(entry.Id));
+        });
+    }
+
+    [Theory]
+    [MemberData(nameof(Cases))]
+    public void Save_LoestOnChangeMitKorrektemEntityTypeAus(
+        string _, string entityType, Func<LiteDbContext, (Guid Id, Action Delete)> setup)
+    {
+        using var db = new LiteDbContext(new MemoryStream());
+        var calls = new List<(string EntityType, string Operation)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, op));
+
+        setup(db);
+
+        Assert.Contains((entityType, "Save"), calls);
+    }
+
+    [Theory]
+    [MemberData(nameof(Cases))]
+    public void Delete_LoestOnChangeMitKorrektemEntityTypeAus(
+        string _, string entityType, Func<LiteDbContext, (Guid Id, Action Delete)> setup)
+    {
+        using var db = new LiteDbContext(new MemoryStream());
+        var (id, delete) = setup(db);
+        var calls = new List<(string EntityType, string EntityId, string Operation)>();
+        db.OnChange = (type, entId, op, payload) => calls.Add((type, entId, op));
+
+        delete();
+
+        Assert.Contains((entityType, id.ToString(), "Delete"), calls);
+    }
+
+    private static object[] Case(string name, string entityType, Func<LiteDbContext, (Guid Id, Action Delete)> setup) =>
+        [name, entityType, setup];
+}

LehrerApp.Data.Tests/ChangeHookTests.cs

@@ -0,0 +1,60 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Data.Repositories;
+using Xunit;
+
+namespace LehrerApp.Data.Tests;
+
+// Vorlage-Test für den OnChange-Hook (Baustein 2), den die übrigen Repositories in den
+// Bausteinen 3/4 auf dieselbe Weise bekommen.
+public sealed class ChangeHookTests
+{
+    private static LiteDbContext NewInMemoryContext() => new(new MemoryStream());
+
+    [Fact]
+    public void StudentRepository_Save_LoestOnChangeMitKorrektenArgumentenAus()
+    {
+        using var db = NewInMemoryContext();
+        var calls = new List<(string EntityType, string EntityId, string Operation, object? Payload)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, id, op, payload));
+        var repo = new StudentRepository(db);
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+
+        repo.Save(student);
+
+        var call = Assert.Single(calls);
+        Assert.Equal("Student", call.EntityType);
+        Assert.Equal(student.Id.ToString(), call.EntityId);
+        Assert.Equal("Save", call.Operation);
+        Assert.Same(student, call.Payload);
+    }
+
+    [Fact]
+    public void StudentRepository_Delete_LoestOnChangeMitNullPayloadAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new StudentRepository(db);
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        repo.Save(student);
+        var calls = new List<(string EntityType, string EntityId, string Operation, object? Payload)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, id, op, payload));
+
+        repo.Delete(student.Id);
+
+        var call = Assert.Single(calls);
+        Assert.Equal("Student", call.EntityType);
+        Assert.Equal(student.Id.ToString(), call.EntityId);
+        Assert.Equal("Delete", call.Operation);
+        Assert.Null(call.Payload);
+    }
+
+    [Fact]
+    public void StudentRepository_Save_OhneGesetztenHook_WirftNicht()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new StudentRepository(db);
+
+        var exception = Record.Exception(() => repo.Save(new Student { FirstName = "Anna", LastName = "Beispiel" }));
+
+        Assert.Null(exception);
+    }
+}

LehrerApp.Data.Tests/RepositoryTests.cs

@@ -267,6 +267,8 @@ public sealed class RepositoryTests
         repo.Save(new SeatingPlan
         {
             GroupId = group.Id, Name = "Standard", Room = "Physik", Rows = 2, Columns = 3,
+            IsBoardAtBottom = true,
+            ColumnGapWidths = [0, 90],
             Assignments = [new SeatAssignment { Row = 1, Column = 2, StudentId = ben.Id }],
         });
 
@@ -274,7 +276,9 @@ public sealed class RepositoryTests
 
         Assert.Equal(2, plans.Count);
         Assert.Contains(plans, p => p.Room == "B204" && p.Assignments.Single().StudentId == anna.Id);
-        Assert.Contains(plans, p => p.Room == "Physik" && p.Assignments.Single().StudentId == ben.Id);
+        Assert.Contains(plans, p => p.Room == "Physik" && p.IsBoardAtBottom
+                                               && p.ColumnGapWidths.SequenceEqual([0, 90])
+                                               && p.Assignments.Single().StudentId == ben.Id);
     }
 
     [Fact]
@@ -299,6 +303,90 @@ public sealed class RepositoryTests
         Assert.Throws<InvalidOperationException>(() => new SeatingPlanRepository(db).Save(plan));
     }
 
+    [Fact]
+    public void SeatingPlanRepository_FiltertAusgeblendetePlaetzeAusserhalbDesRasters()
+    {
+        using var db = NewInMemoryContext();
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        new GroupRepository(db).Save(group);
+        var plan = new SeatingPlan
+        {
+            GroupId = group.Id, Name = "Standard", Rows = 2, Columns = 2,
+            HiddenSeats =
+            [
+                new HiddenSeat { Row = 0, Column = 1 },
+                new HiddenSeat { Row = 5, Column = 5 }, // außerhalb des 2x2-Rasters
+            ],
+        };
+
+        new SeatingPlanRepository(db).Save(plan);
+
+        var saved = db.SeatingPlans.FindById(plan.Id);
+        Assert.NotNull(saved);
+        var hidden = Assert.Single(saved.HiddenSeats);
+        Assert.Equal((0, 1), (hidden.Row, hidden.Column));
+    }
+
+    /// Ein ausgeblendeter Platz darf nie belegt sein - die Zuordnung eines Schülers ist wertvoller
+    /// als das Ausblenden. SeatingPlanTabViewModel verhindert das Belegen ausgeblendeter Plätze
+    /// bereits in der UI; diese Absicherung greift für Daten, die diesen Weg umgehen (z. B. Sync
+    /// von einem älteren Client).
+    [Fact]
+    public void SeatingPlanRepository_BlendetPlatzWiederEinWennErBelegtWird()
+    {
+        using var db = NewInMemoryContext();
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        new GroupRepository(db).Save(group);
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        db.Students.Insert(student);
+        db.Memberships.Insert(new GroupMembership { GroupId = group.Id, StudentId = student.Id });
+        var plan = new SeatingPlan
+        {
+            GroupId = group.Id, Name = "Standard", Rows = 2, Columns = 2,
+            HiddenSeats = [new HiddenSeat { Row = 0, Column = 0 }],
+            Assignments = [new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id }],
+        };
+
+        new SeatingPlanRepository(db).Save(plan);
+
+        var saved = db.SeatingPlans.FindById(plan.Id);
+        Assert.NotNull(saved);
+        Assert.Empty(saved.HiddenSeats);
+        Assert.Equal(student.Id, saved.Assignments.Single().StudentId);
+    }
+
+    [Fact]
+    public void SeatingPlanRepository_AktualisiertPlanMitFehlendemOptionalenRaum()
+    {
+        using var db = NewInMemoryContext();
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        new GroupRepository(db).Save(group);
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        db.Students.Insert(student);
+        db.Memberships.Insert(new GroupMembership { GroupId = group.Id, StudentId = student.Id });
+        var plan = new SeatingPlan
+        {
+            GroupId = group.Id,
+            Name = "Standard",
+            Room = null!,
+            Rows = 2,
+            Columns = 2,
+        };
+        db.SeatingPlans.Insert(plan);
+        plan.Assignments.Add(new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id });
+
+        new SeatingPlanRepository(db).Save(plan);
+
+        var saved = db.SeatingPlans.FindById(plan.Id);
+        Assert.NotNull(saved);
+        Assert.True(string.IsNullOrEmpty(saved.Room));
+        Assert.Equal(student.Id, saved.Assignments.Single().StudentId);
+
+        // LiteDB may materialize the empty optional value as null again. A later
+        // drag/drop save must therefore remain safe as well.
+        new SeatingPlanRepository(db).Save(saved);
+    }
+
     [Fact]
     public void GroupMembershipRepository_Save_LehntZweiteZuordnungFuerGleichesPaarAb()
     {
@@ -652,6 +740,25 @@ public sealed class RepositoryTests
         Assert.Equal(2, result[2].LessonNumber);
     }
 
+    [Fact]
+    public void LessonRepository_DeleteRaeumtAnhaengeAusDerAttachmentAblageAuf()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new LessonRepository(db);
+        var lesson = new Lesson
+        {
+            UnitId = Guid.NewGuid(), GroupId = Guid.NewGuid(),
+            Attachments = [new DocumentAttachment { StorageId = "gbu-1", FileName = "gbu.pdf" }],
+        };
+        db.Attachments.Upload("gbu-1", "gbu.pdf", new MemoryStream([1, 2, 3]));
+        repo.Save(lesson);
+
+        repo.Delete(lesson.Id);
+
+        Assert.Null(db.Lessons.FindById(lesson.Id));
+        Assert.False(db.Attachments.Exists("gbu-1"));
+    }
+
     // ── ShorthandCodeRepository ───────────────────────────────────────────────
 
     [Fact]
@@ -847,4 +954,238 @@ public sealed class RepositoryTests
         Assert.Single(result);
         Assert.Equal("Vertretung 8a", result[0].Description);
     }
+
+    [Fact]
+    public void SubstitutionEntryRepository_GetByExternalId_FindetAutomatischErzeugtenEintrag()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new SubstitutionEntryRepository(db);
+        repo.Save(new SubstitutionEntry { Date = new DateOnly(2026, 3, 12), Kind = SubstitutionKind.Cancelled, PeriodNumber = 3, ExternalId = "38818-2004584-2004587" });
+        repo.Save(new SubstitutionEntry { Date = new DateOnly(2026, 3, 13), Kind = SubstitutionKind.Cancelled, PeriodNumber = 4 });
+
+        var result = repo.GetByExternalId("38818-2004584-2004587");
+
+        Assert.NotNull(result);
+        Assert.Equal(new DateOnly(2026, 3, 12), result!.Date);
+        Assert.Null(repo.GetByExternalId("unbekannt"));
+    }
+
+    // ── UntisSnapshotRepository / UntisSlotMappingRepository ─────────────────
+
+    [Fact]
+    public void UntisSnapshotRepository_SaveUndGetAll_RoundTrip()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisSnapshotRepository(db);
+        repo.Save(new UntisSnapshotEntry
+        {
+            Uid = "38926-2013812-2013815", Date = new DateOnly(2026, 8, 17),
+            StartTime = new TimeOnly(7, 50), EndTime = new TimeOnly(9, 20),
+            Summary = "SOL", Location = "Medien", Description = "10c HED",
+        });
+
+        var result = repo.GetAll();
+
+        Assert.Single(result);
+        Assert.Equal("38926-2013812-2013815", result[0].Uid);
+        Assert.Equal("SOL", result[0].Summary);
+    }
+
+    [Fact]
+    public void UntisSnapshotRepository_Save_AktualisiertVorhandenenEintrag()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisSnapshotRepository(db);
+        var entry = new UntisSnapshotEntry { Uid = "38926-2013812-2013815", Location = "Medien" };
+        repo.Save(entry);
+
+        entry.Location = "A1";
+        entry.Status = "CANCELLED";
+        repo.Save(entry);
+
+        var result = Assert.Single(repo.GetAll());
+        Assert.Equal("A1", result.Location);
+        Assert.Equal("CANCELLED", result.Status);
+    }
+
+    [Fact]
+    public void UntisSlotMappingRepository_SaveDeleteUndGetAll_RoundTrip()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisSlotMappingRepository(db);
+        var groupId = Guid.NewGuid();
+        var mapping = new UntisSlotMapping
+        {
+            Weekday = DayOfWeek.Monday, StartTime = new TimeOnly(7, 50),
+            Summary = "SOL", ClassToken = "10c", GroupId = groupId, Confirmed = true,
+        };
+        repo.Save(mapping);
+
+        Assert.Single(repo.GetAll());
+
+        repo.Delete(mapping.Id);
+
+        Assert.Empty(repo.GetAll());
+    }
+
+    // ── AnnualPlanEventRepository ────────────────────────────────────────────
+
+    [Fact]
+    public void AnnualPlanEventRepository_GetByRange_FindetAuchUeberlappendeMehrtagstermine()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new AnnualPlanEventRepository(db);
+        repo.Save(new AnnualPlanEvent
+        {
+            ExternalId = "fahrt", Title = "Klassenfahrt",
+            StartDate = new DateOnly(2026, 6, 15), EndDate = new DateOnly(2026, 6, 19), IsAllDay = true,
+        });
+        repo.Save(new AnnualPlanEvent
+        {
+            ExternalId = "spaeter", Title = "Sommerfest",
+            StartDate = new DateOnly(2026, 7, 1), EndDate = new DateOnly(2026, 7, 1), IsAllDay = true,
+        });
+
+        var result = repo.GetByRange(new DateOnly(2026, 6, 17), new DateOnly(2026, 6, 17));
+
+        Assert.Single(result);
+        Assert.Equal("fahrt", result[0].ExternalId);
+        Assert.Equal("fahrt", repo.GetByExternalId("fahrt")!.ExternalId);
+    }
+
+    // ── UntisAbsenceCacheRepository / UntisClassRegisterCacheRepository / UntisCacheFetchStateRepository ──
+
+    [Fact]
+    public void UntisAbsenceCacheRepository_GetByClassAndRange_FiltertKlasseUndDatumsbereich()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisAbsenceCacheRepository(db);
+        repo.InsertRange(
+        [
+            new UntisAbsenceCacheEntry { ClassName = "10c", StudentName = "Max", Date = 20260824 },
+            new UntisAbsenceCacheEntry { ClassName = "10c", StudentName = "Max", Date = 20260901 },
+            new UntisAbsenceCacheEntry { ClassName = "10d", StudentName = "Ben", Date = 20260824 },
+        ]);
+
+        var result = repo.GetByClassAndRange("10c", 20260801, 20260831);
+
+        Assert.Single(result);
+        Assert.Equal(20260824, result[0].Date);
+    }
+
+    [Fact]
+    public void UntisAbsenceCacheRepository_ReplaceRange_ErsetztNurDenAngegebenenBereich()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisAbsenceCacheRepository(db);
+        repo.InsertRange(
+        [
+            new UntisAbsenceCacheEntry { ClassName = "10c", StudentName = "Alt", Date = 20260810 },
+            new UntisAbsenceCacheEntry { ClassName = "10c", StudentName = "Alt", Date = 20260901 },
+        ]);
+
+        repo.ReplaceRange("10c", 20260801, 20260831,
+            [new UntisAbsenceCacheEntry { ClassName = "10c", StudentName = "Neu", Date = 20260815 }]);
+
+        var inRange = repo.GetByClassAndRange("10c", 20260801, 20260831);
+        var untouched = repo.GetByClassAndRange("10c", 20260901, 20260901);
+        Assert.Equal("Neu", Assert.Single(inRange).StudentName);
+        Assert.Single(untouched);
+    }
+
+    [Fact]
+    public void UntisClassRegisterCacheRepository_InsertUndGetByClassAndRange_RoundTrip()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisClassRegisterCacheRepository(db);
+        repo.InsertRange([new UntisClassRegisterCacheEntry
+        {
+            ClassName = "10c", Date = 20260824, StudentName = "Max", Subject = "Deu",
+            TeacherUsername = "mueller", CategoryName = "Positiv",
+        }]);
+
+        var result = Assert.Single(repo.GetByClassAndRange("10c", 20260801, 20260831));
+        Assert.Equal("mueller", result.TeacherUsername);
+    }
+
+    [Fact]
+    public void UntisStudentRosterCacheRepository_ReplaceAll_ErsetztNurDieAngegebeneKlasse()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisStudentRosterCacheRepository(db);
+        repo.ReplaceAll("10c", [new UntisStudentRosterCacheEntry { ClassName = "10c", DisplayName = "Alt" }]);
+        repo.ReplaceAll("10d", [new UntisStudentRosterCacheEntry { ClassName = "10d", DisplayName = "Andere Klasse" }]);
+
+        repo.ReplaceAll("10c", [new UntisStudentRosterCacheEntry { ClassName = "10c", DisplayName = "Neu", ExternKey = 42 }]);
+
+        var tenC = repo.GetByClass("10c");
+        var tenD = repo.GetByClass("10d");
+        Assert.Equal("Neu", Assert.Single(tenC).DisplayName);
+        Assert.Equal(42, tenC[0].ExternKey);
+        Assert.Single(tenD); // andere Klasse bleibt unberührt
+    }
+
+    [Fact]
+    public void UntisCacheFetchStateRepository_SaveUndGet_RoundTripJeKlasseUndArt()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new UntisCacheFetchStateRepository(db);
+        var fetchedAt = new DateTime(2026, 8, 25, 10, 0, 0, DateTimeKind.Utc);
+        repo.Save(new UntisCacheFetchState
+        {
+            ClassName = "10c", Kind = UntisCacheKind.Absences,
+            HotWindowFetchedAt = fetchedAt, ColdCoverageStartDate = 20260101,
+        });
+
+        var result = repo.Get("10c", UntisCacheKind.Absences);
+        var missing = repo.Get("10c", UntisCacheKind.ClassRegister);
+
+        Assert.NotNull(result);
+        // LiteDB rundet Kind Utc->Local, verschiebt dabei die Ticks um denselben Zeitpunkt zu
+        // erhalten (siehe CLAUDE.md) - deshalb erst beide Seiten auf UTC normalisieren.
+        Assert.Equal(fetchedAt.ToUniversalTime(), result!.HotWindowFetchedAt!.Value.ToUniversalTime());
+        Assert.Equal(20260101, result.ColdCoverageStartDate);
+        Assert.Null(missing);
+    }
+
+    // ── WorkTaskRepository ────────────────────────────────────────────────────
+
+    [Fact]
+    public void WorkTaskRepository_GetByGroup_FindetNurAufgabenDieserGruppe()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new WorkTaskRepository(db);
+        var groupId = Guid.NewGuid();
+        repo.Save(new WorkTask { Title = "Klassenaufgabe", GroupId = groupId });
+        repo.Save(new WorkTask { Title = "Andere Gruppe", GroupId = Guid.NewGuid() });
+        repo.Save(new WorkTask { Title = "Ohne Gruppe", GroupId = null });
+
+        var result = repo.GetByGroup(groupId);
+
+        Assert.Single(result);
+        Assert.Equal("Klassenaufgabe", result[0].Title);
+    }
+
+    [Fact]
+    public void WorkTaskRepository_SaveUndLoad_RundetPrioritaetUndChecklistItemsRoundTrip()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new WorkTaskRepository(db);
+        var studentId = Guid.NewGuid();
+        var task = new WorkTask
+        {
+            Title = "Material einsammeln",
+            Priority = TaskPriority.High,
+            ChecklistItems = [new ChecklistItem { Label = "Anna", StudentId = studentId, IsDone = true }],
+        };
+
+        repo.Save(task);
+        var loaded = repo.GetAll().Single(t => t.Id == task.Id);
+
+        Assert.Equal(TaskPriority.High, loaded.Priority);
+        var item = Assert.Single(loaded.ChecklistItems);
+        Assert.Equal("Anna", item.Label);
+        Assert.Equal(studentId, item.StudentId);
+        Assert.True(item.IsDone);
+    }
 }

LehrerApp.Data.Tests/TrashTests.cs

@@ -0,0 +1,181 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Data.Repositories;
+using Xunit;
+
+namespace LehrerApp.Data.Tests;
+
+public sealed class TrashTests
+{
+    private static LiteDbContext NewInMemoryContext() => new(new MemoryStream());
+
+    [Fact]
+    public void GradeRepository_DeleteVerschiebtInDenPapierkorbUndRestoreStelltWiederHer()
+    {
+        using var db = NewInMemoryContext();
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        new GroupRepository(db).Save(group);
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        db.Students.Insert(student);
+        var repo = new GradeRepository(db);
+        var grade = new Grade
+        {
+            StudentId = student.Id, GroupId = group.Id, Category = GradeCategory.Oral,
+            Value = "2", Date = new DateOnly(2026, 3, 4),
+        };
+        repo.Save(grade);
+
+        repo.Delete(grade.Id);
+
+        Assert.Null(db.Grades.FindById(grade.Id));
+        var trashed = db.TrashedItems.FindAll().Single();
+        Assert.Equal(nameof(Grade), trashed.EntityType);
+        Assert.Equal("Note 2 (04.03.2026)", trashed.Summary);
+
+        repo.Restore(trashed.Id);
+
+        var restored = db.Grades.FindById(grade.Id);
+        Assert.NotNull(restored);
+        Assert.Equal("2", restored!.Value);
+        Assert.Empty(db.TrashedItems.FindAll());
+    }
+
+    [Fact]
+    public void GradeRepository_RestoreMitUnbekannterTrashIdTutNichts()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new GradeRepository(db);
+
+        repo.Restore(Guid.NewGuid());
+
+        Assert.Empty(db.Grades.FindAll());
+    }
+
+    [Fact]
+    public void WorkTaskRepository_DeleteVerschiebtInDenPapierkorbUndRestoreStelltWiederHer()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new WorkTaskRepository(db);
+        var task = new WorkTask { Title = "Elternbrief schreiben", Category = TaskCategory.Admin };
+        repo.Save(task);
+
+        repo.Delete(task.Id);
+
+        Assert.Null(db.Tasks.FindById(task.Id));
+        Assert.Equal("Elternbrief schreiben", db.TrashedItems.FindAll().Single().Summary);
+
+        repo.Restore(db.TrashedItems.FindAll().Single().Id);
+
+        Assert.NotNull(db.Tasks.FindById(task.Id));
+    }
+
+    [Fact]
+    public void TimeEntryRepository_DeleteVerschiebtInDenPapierkorbUndRestoreStelltWiederHer()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new TimeEntryRepository(db);
+        var entry = new TimeEntry { Category = "Korrektur", Date = new DateOnly(2026, 3, 4), DurationMinutes = 45 };
+        repo.Save(entry);
+
+        repo.Delete(entry.Id);
+
+        Assert.Null(db.TimeEntries.FindById(entry.Id));
+        Assert.Equal("45 Min. Korrektur (04.03.2026)", db.TrashedItems.FindAll().Single().Summary);
+
+        repo.Restore(db.TrashedItems.FindAll().Single().Id);
+
+        Assert.NotNull(db.TimeEntries.FindById(entry.Id));
+    }
+
+    [Fact]
+    public void SeatingPlanRepository_DeleteVerschiebtInDenPapierkorbUndRestoreStelltWiederHer()
+    {
+        using var db = NewInMemoryContext();
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        new GroupRepository(db).Save(group);
+        var repo = new SeatingPlanRepository(db);
+        var plan = new SeatingPlan { GroupId = group.Id, Name = "Standard", Room = "B204", Rows = 2, Columns = 2 };
+        repo.Save(plan);
+
+        repo.Delete(plan.Id);
+
+        Assert.Null(db.SeatingPlans.FindById(plan.Id));
+        Assert.Equal("Standard (Raum B204)", db.TrashedItems.FindAll().Single().Summary);
+
+        repo.Restore(db.TrashedItems.FindAll().Single().Id);
+
+        Assert.NotNull(db.SeatingPlans.FindById(plan.Id));
+    }
+
+    [Fact]
+    public void SeatingPlanRepository_DeleteOhneRaumNutztNurDenNamenAlsSummary()
+    {
+        using var db = NewInMemoryContext();
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2025/26" };
+        new GroupRepository(db).Save(group);
+        var repo = new SeatingPlanRepository(db);
+        var plan = new SeatingPlan { GroupId = group.Id, Name = "Standard", Rows = 2, Columns = 2 };
+        repo.Save(plan);
+
+        repo.Delete(plan.Id);
+
+        Assert.Equal("Standard", db.TrashedItems.FindAll().Single().Summary);
+    }
+
+    [Fact]
+    public void GradingKeyTemplateRepository_DeleteVerschiebtInDenPapierkorbUndRestoreStelltWiederHer()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new GradingKeyTemplateRepository(db);
+        var template = new GradingKeyTemplate { Name = "Standard 1-6", GradingSystem = GradingSystem.Grades1To6 };
+        repo.Save(template);
+
+        repo.Delete(template.Id);
+
+        Assert.Null(db.GradingKeyTemplates.FindById(template.Id));
+        Assert.Equal("Standard 1-6", db.TrashedItems.FindAll().Single().Summary);
+
+        repo.Restore(db.TrashedItems.FindAll().Single().Id);
+
+        Assert.NotNull(db.GradingKeyTemplates.FindById(template.Id));
+    }
+
+    [Fact]
+    public void TrashRepository_GetAllSortiertNeuesteZuerst()
+    {
+        using var db = NewInMemoryContext();
+        db.TrashedItems.Insert(new TrashedItem
+        {
+            EntityType = nameof(WorkTask), Summary = "Älter", DeletedAt = DateTime.UtcNow.AddDays(-2),
+        });
+        db.TrashedItems.Insert(new TrashedItem
+        {
+            EntityType = nameof(WorkTask), Summary = "Neuer", DeletedAt = DateTime.UtcNow,
+        });
+
+        var all = new TrashRepository(db).GetAll();
+
+        Assert.Equal(2, all.Count);
+        Assert.Equal("Neuer", all[0].Summary);
+        Assert.Equal("Älter", all[1].Summary);
+    }
+
+    [Fact]
+    public void TrashRepository_PurgeOlderThanEntferntNurAeltereEintraege()
+    {
+        using var db = NewInMemoryContext();
+        db.TrashedItems.Insert(new TrashedItem
+        {
+            EntityType = nameof(WorkTask), Summary = "Alt", DeletedAt = DateTime.UtcNow.AddDays(-40),
+        });
+        db.TrashedItems.Insert(new TrashedItem
+        {
+            EntityType = nameof(WorkTask), Summary = "Frisch", DeletedAt = DateTime.UtcNow,
+        });
+
+        new TrashRepository(db).PurgeOlderThan(DateTime.UtcNow.AddDays(-30));
+
+        var remaining = db.TrashedItems.FindAll().ToList();
+        Assert.Single(remaining);
+        Assert.Equal("Frisch", remaining[0].Summary);
+    }
+}

LehrerApp.Data/AssemblyInfo.cs

@@ -0,0 +1,5 @@
+using System.Runtime.CompilerServices;
+
+// EventApplier (Baustein 5) muss dieselben internen Kaskaden-Hilfsmethoden auf LiteDbContext
+// wiederverwenden können wie die Repositories selbst (siehe LiteDbContext.CascadeDelete*).
+[assembly: InternalsVisibleTo("LehrerApp.Sync")]

LehrerApp.Data/LiteDbContext.cs

@@ -3,6 +3,10 @@ using LehrerApp.Core.Models;
 
 namespace LehrerApp.Data;
 
+/// Wird nach jedem Save/Delete einer Entität aufgerufen; payload ist die gespeicherte Entität
+/// bzw. null bei Delete. Sync-agnostisch – siehe <see cref="LiteDbContext.OnChange"/>.
+public delegate void ChangeHandler(string entityType, string entityId, string operation, object? payload);
+
 /// <summary>
 /// Zentrale LiteDB-Verbindung. Singleton – eine Datei = ein Nutzer.
 /// </summary>
@@ -62,11 +66,162 @@ public class LiteDbContext : IDisposable
     public ILiteCollection<SchoolHoliday>  SchoolHolidays  => _db.GetCollection<SchoolHoliday>("school_holidays");
     public ILiteCollection<SupervisionDuty> SupervisionDuties => _db.GetCollection<SupervisionDuty>("supervision_duties");
     public ILiteCollection<SubstitutionEntry> SubstitutionEntries => _db.GetCollection<SubstitutionEntry>("substitution_entries");
+    public ILiteCollection<UntisSnapshotEntry> UntisSnapshotEntries => _db.GetCollection<UntisSnapshotEntry>("untis_snapshot_entries");
+    public ILiteCollection<UntisSlotMapping> UntisSlotMappings => _db.GetCollection<UntisSlotMapping>("untis_slot_mappings");
+    public ILiteCollection<UntisAbsenceCacheEntry> UntisAbsenceCache => _db.GetCollection<UntisAbsenceCacheEntry>("untis_absence_cache");
+    public ILiteCollection<UntisClassRegisterCacheEntry> UntisClassRegisterCache => _db.GetCollection<UntisClassRegisterCacheEntry>("untis_classregister_cache");
+    public ILiteCollection<UntisCacheFetchState> UntisCacheFetchStates => _db.GetCollection<UntisCacheFetchState>("untis_cache_fetch_state");
+    public ILiteCollection<UntisStudentRosterCacheEntry> UntisStudentRosterCache => _db.GetCollection<UntisStudentRosterCacheEntry>("untis_student_roster_cache");
+    public ILiteCollection<AnnualPlanEvent> AnnualPlanEvents => _db.GetCollection<AnnualPlanEvent>("annual_plan_events");
+    public ILiteCollection<TrashedItem>    TrashedItems    => _db.GetCollection<TrashedItem>("trash");
 
     public void Checkpoint() => _db.Checkpoint();
 
     public int SchemaVersion => ReadSchemaVersion();
 
+    /// Wird von den Repositories nach jedem Save/Delete aufgerufen (payload = die gespeicherte
+    /// Entität bzw. null bei Delete). Bewusst hier statt in LehrerApp.Sync definiert, damit Data
+    /// weiterhin ohne Verweis auf Sync auskommt — die eigentliche Sync-Anbindung setzt diesen
+    /// Hook von außen (siehe AppBootstrapper).
+    public ChangeHandler? OnChange { get; set; }
+
+    /// Führt dieselbe Kaskade wie <c>GroupRepository.Delete</c> aus. Hier auf dem Context statt
+    /// im Repository, damit ein später eingehendes Sync-Ereignis (Baustein 5) dieselbe Kaskade
+    /// nachvollziehen kann, ohne über die Repository-Save/Delete-Methoden (und damit erneut über
+    /// <see cref="OnChange"/>) zu laufen.
+    internal void CascadeDeleteGroup(Guid id)
+    {
+        ExecuteInTransaction(() =>
+        {
+            foreach (var membership in Memberships.Find(e => e.GroupId == id).ToList())
+                Memberships.Delete(membership.Id);
+
+            foreach (var exam in Exams.Find(e => e.GroupId == id).ToList())
+            {
+                foreach (var result in ExamResults.Find(r => r.ExamId == exam.Id).ToList())
+                    ExamResults.Delete(result.Id);
+                Exams.Delete(exam.Id);
+            }
+
+            foreach (var grade in Grades.Find(g => g.GroupId == id).ToList())
+                Grades.Delete(grade.Id);
+
+            foreach (var reportGrade in ReportGrades.Find(g => g.GroupId == id).ToList())
+                ReportGrades.Delete(reportGrade.Id);
+
+            foreach (var scheme in GradingSchemes.Find(s => s.GroupId == id).ToList())
+                GradingSchemes.Delete(scheme.Id);
+
+            foreach (var unit in Units.Find(u => u.GroupId == id).ToList())
+            {
+                foreach (var lesson in Lessons.Find(l => l.UnitId == unit.Id).ToList())
+                    Lessons.Delete(lesson.Id);
+                Units.Delete(unit.Id);
+            }
+
+            foreach (var lesson in Lessons.Find(l => l.GroupId == id).ToList())
+                Lessons.Delete(lesson.Id);
+
+            foreach (var session in ParticipationSessions.Find(s => s.GroupId == id).ToList())
+            {
+                foreach (var entry in ParticipationEntries.Find(e => e.SessionId == session.Id).ToList())
+                    ParticipationEntries.Delete(entry.Id);
+                ParticipationSessions.Delete(session.Id);
+            }
+
+            foreach (var aspect in ParticipationAspects.Find(a => a.GroupId == id).ToList())
+                ParticipationAspects.Delete(aspect.Id);
+
+            foreach (var section in ParticipationSections.Find(s => s.GroupId == id).ToList())
+                ParticipationSections.Delete(section.Id);
+
+            foreach (var plan in SeatingPlans.Find(p => p.GroupId == id).ToList())
+                SeatingPlans.Delete(plan.Id);
+
+            // Dokumentation und Arbeitszeit sind historische Nachweise. Sie bleiben erhalten,
+            // werden aber von der nicht mehr existierenden Lerngruppe entkoppelt.
+            foreach (var documentation in Documentation.Find(d => d.GroupId == id).ToList())
+            {
+                documentation.GroupId = null;
+                documentation.UpdatedAt = DateTime.UtcNow;
+                Documentation.Update(documentation);
+            }
+
+            foreach (var task in Tasks.Find(t => t.GroupId == id).ToList())
+            {
+                task.GroupId = null;
+                task.UpdatedAt = DateTime.UtcNow;
+                Tasks.Update(task);
+            }
+
+            foreach (var timeEntry in TimeEntries.Find(t => t.GroupId == id).ToList())
+            {
+                timeEntry.GroupId = null;
+                TimeEntries.Update(timeEntry);
+            }
+
+            Groups.Delete(id);
+        });
+    }
+
+    /// Führt dieselbe Kaskade wie <c>ExamRepository.Delete</c> aus (siehe <see cref="CascadeDeleteGroup"/>).
+    internal void CascadeDeleteExam(Guid id)
+    {
+        foreach (var result in ExamResults.Find(r => r.ExamId == id).ToList())
+            ExamResults.Delete(result.Id);
+        Exams.Delete(id);
+    }
+
+    /// Führt dieselbe Kaskade wie <c>ParticipationSessionRepository.Delete</c> aus (siehe
+    /// <see cref="CascadeDeleteGroup"/>).
+    internal void CascadeDeleteParticipationSession(Guid id)
+    {
+        ParticipationSessions.Delete(id);
+        foreach (var e in ParticipationEntries.Find(e => e.SessionId == id).ToList())
+            ParticipationEntries.Delete(e.Id);
+    }
+
+    /// Führt dieselbe Kaskade wie <c>DocumentationRepository.HardDelete</c> aus (siehe
+    /// <see cref="CascadeDeleteGroup"/>).
+    internal void CascadeHardDeleteDocumentation(Guid id)
+    {
+        if (Documentation.FindById(id) is { } doc)
+            foreach (var attachment in doc.Attachments) Attachments.Delete(attachment.StorageId);
+        Documentation.Delete(id);
+    }
+
+    // ── Papierkorb (14.3) ────────────────────────────────────────────────────
+    //
+    // Absichtlich hier statt in einer eigenen TrashRepository-Konstruktorabhängigkeit: jede
+    // Fach-Repository hat db (LiteDbContext) bereits im Konstruktor, ein zusätzlicher Papierkorb-
+    // Parameter wäre nur Boilerplate. Snapshot/Restore sind generisch über den Modelltyp, die
+    // eigentliche Wiederherstellung ruft aber bewusst die jeweilige Repository.Save-Methode auf
+    // (nicht direkt die LiteDB-Collection) — nur die kennt Validierung und OnChange-Ereignis für
+    // ihr Modell.
+
+    /// <summary>Legt einen Papierkorb-Eintrag an, BEVOR die aufrufende Repository die Entität aus
+    /// ihrer eigentlichen Collection löscht.</summary>
+    internal void MoveToTrash<T>(string entityType, Guid entityId, T entity, string summary) =>
+        TrashedItems.Insert(new TrashedItem
+        {
+            EntityType = entityType,
+            EntityId   = entityId,
+            Snapshot   = System.Text.Json.JsonSerializer.Serialize(entity),
+            Summary    = summary,
+        });
+
+    /// <summary>Liefert die gelöschte Entität aus dem Papierkorb-Eintrag zurück und entfernt den
+    /// Eintrag - unabhängig davon, ob die aufrufende Repository den Schnappschuss danach
+    /// erfolgreich speichert (ein fehlgeschlagenes Restore soll nicht endlos wiederholbar sein,
+    /// derselbe Datensatz landet notfalls einfach nicht wieder im Papierkorb).</summary>
+    internal T? RestoreFromTrash<T>(Guid trashId)
+    {
+        var item = TrashedItems.FindById(trashId);
+        if (item is null) return default;
+        TrashedItems.Delete(trashId);
+        return System.Text.Json.JsonSerializer.Deserialize<T>(item.Snapshot);
+    }
+
     internal void ExecuteInTransaction(Action action)
     {
         _db.BeginTrans();
@@ -381,6 +536,16 @@ public class LiteDbContext : IDisposable
         SupervisionDuties.EnsureIndex("ux_supervision_weekday_period",
             BsonExpression.Create("STRING($.Weekday) + ':' + STRING($.AfterPeriod)"), unique: true);
         SubstitutionEntries.EnsureIndex(x => x.Date);
+        AnnualPlanEvents.EnsureIndex(x => x.ExternalId, unique: true);
+        AnnualPlanEvents.EnsureIndex(x => x.StartDate);
+        AnnualPlanEvents.EnsureIndex(x => x.EndDate);
+        UntisAbsenceCache.EnsureIndex(x => x.ClassName);
+        UntisAbsenceCache.EnsureIndex(x => x.Date);
+        UntisClassRegisterCache.EnsureIndex(x => x.ClassName);
+        UntisClassRegisterCache.EnsureIndex(x => x.Date);
+        UntisStudentRosterCache.EnsureIndex(x => x.ClassName);
+        UntisCacheFetchStates.EnsureIndex("ux_class_kind",
+            BsonExpression.Create("STRING($.ClassName) + ':' + STRING($.Kind)"), unique: true);
     }
 
     public void Dispose() => _db.Dispose();

LehrerApp.Data/Repositories/AllRepositories.cs

@@ -35,7 +35,14 @@ public class StudentRepository(LiteDbContext db) : IStudentRepository
         db.ReportGrades.Count(g => g.StudentId == studentId),
         db.ParticipationEntries.Count(e => e.StudentId == studentId),
         db.Documentation.Count(d => d.StudentId == studentId));
-    public void Save(Student s) { s.UpdatedAt = DateTime.UtcNow; db.Students.Upsert(s); }
+    public void Save(Student s)
+    {
+        s.ExternalIds ??= [];
+        s.Contacts ??= [];
+        s.UpdatedAt = DateTime.UtcNow;
+        db.Students.Upsert(s);
+        db.OnChange?.Invoke(nameof(Student), s.Id.ToString(), "Save", s);
+    }
     public void Delete(Guid id)
     {
         var references = GetReferenceSummary(id);
@@ -43,6 +50,7 @@ public class StudentRepository(LiteDbContext db) : IStudentRepository
             throw new InvalidOperationException(
                 "Der Schüler besitzt verknüpfte Daten und kann nur deaktiviert werden.");
         db.Students.Delete(id);
+        db.OnChange?.Invoke(nameof(Student), id.ToString(), "Delete", null);
     }
 }
 
@@ -73,81 +81,13 @@ public class GroupRepository(LiteDbContext db) : IGroupRepository
             throw new InvalidOperationException("Das gewählte Fach existiert nicht mehr.");
         g.UpdatedAt = DateTime.UtcNow;
         db.Groups.Upsert(g);
+        db.OnChange?.Invoke(nameof(LearningGroup), g.Id.ToString(), "Save", g);
     }
     public void Delete(Guid id)
     {
         ArchivedGroupWriteGuard.EnsureActive(db, id);
-        db.ExecuteInTransaction(() =>
+        db.CascadeDeleteGroup(id);
-        {
+        db.OnChange?.Invoke(nameof(LearningGroup), id.ToString(), "Delete", null);
-            foreach (var membership in db.Memberships.Find(e => e.GroupId == id).ToList())
-                db.Memberships.Delete(membership.Id);
-
-            foreach (var exam in db.Exams.Find(e => e.GroupId == id).ToList())
-            {
-                foreach (var result in db.ExamResults.Find(r => r.ExamId == exam.Id).ToList())
-                    db.ExamResults.Delete(result.Id);
-                db.Exams.Delete(exam.Id);
-            }
-
-            foreach (var grade in db.Grades.Find(g => g.GroupId == id).ToList())
-                db.Grades.Delete(grade.Id);
-
-            foreach (var reportGrade in db.ReportGrades.Find(g => g.GroupId == id).ToList())
-                db.ReportGrades.Delete(reportGrade.Id);
-
-            foreach (var scheme in db.GradingSchemes.Find(s => s.GroupId == id).ToList())
-                db.GradingSchemes.Delete(scheme.Id);
-
-            foreach (var unit in db.Units.Find(u => u.GroupId == id).ToList())
-            {
-                foreach (var lesson in db.Lessons.Find(l => l.UnitId == unit.Id).ToList())
-                    db.Lessons.Delete(lesson.Id);
-                db.Units.Delete(unit.Id);
-            }
-
-            foreach (var lesson in db.Lessons.Find(l => l.GroupId == id).ToList())
-                db.Lessons.Delete(lesson.Id);
-
-            foreach (var session in db.ParticipationSessions.Find(s => s.GroupId == id).ToList())
-            {
-                foreach (var entry in db.ParticipationEntries.Find(e => e.SessionId == session.Id).ToList())
-                    db.ParticipationEntries.Delete(entry.Id);
-                db.ParticipationSessions.Delete(session.Id);
-            }
-
-            foreach (var aspect in db.ParticipationAspects.Find(a => a.GroupId == id).ToList())
-                db.ParticipationAspects.Delete(aspect.Id);
-
-            foreach (var section in db.ParticipationSections.Find(s => s.GroupId == id).ToList())
-                db.ParticipationSections.Delete(section.Id);
-
-            foreach (var plan in db.SeatingPlans.Find(p => p.GroupId == id).ToList())
-                db.SeatingPlans.Delete(plan.Id);
-
-            // Dokumentation und Arbeitszeit sind historische Nachweise. Sie bleiben erhalten,
-            // werden aber von der nicht mehr existierenden Lerngruppe entkoppelt.
-            foreach (var documentation in db.Documentation.Find(d => d.GroupId == id).ToList())
-            {
-                documentation.GroupId = null;
-                documentation.UpdatedAt = DateTime.UtcNow;
-                db.Documentation.Update(documentation);
-            }
-
-            foreach (var task in db.Tasks.Find(t => t.GroupId == id).ToList())
-            {
-                task.GroupId = null;
-                task.UpdatedAt = DateTime.UtcNow;
-                db.Tasks.Update(task);
-            }
-
-            foreach (var timeEntry in db.TimeEntries.Find(t => t.GroupId == id).ToList())
-            {
-                timeEntry.GroupId = null;
-                db.TimeEntries.Update(timeEntry);
-            }
-
-            db.Groups.Delete(id);
-        });
     }
 }
 
@@ -162,8 +102,13 @@ public class SeatingPlanRepository(LiteDbContext db) : ISeatingPlanRepository
     public void Save(SeatingPlan plan)
     {
         ArchivedGroupWriteGuard.EnsureActive(db, plan.GroupId);
-        plan.Name = plan.Name.Trim();
+        // LiteDB can deserialize missing/legacy optional string fields as null even
-        plan.Room = plan.Room.Trim();
+        // though the current model initializes them with an empty string.
+        plan.Name = plan.Name?.Trim() ?? "";
+        plan.Room = plan.Room?.Trim() ?? "";
+        plan.Assignments ??= [];
+        plan.ColumnGapWidths ??= [];
+        plan.HiddenSeats ??= [];
         if (plan.Name.Length == 0)
             throw new ArgumentException("Der Name des Sitzplans darf nicht leer sein.");
         if (plan.Rows is < 1 or > 10 || plan.Columns is < 1 or > 10)
@@ -171,6 +116,12 @@ public class SeatingPlanRepository(LiteDbContext db) : ISeatingPlanRepository
         if (db.Groups.FindById(plan.GroupId) is null)
             throw new InvalidOperationException("Die zugehörige Lerngruppe existiert nicht.");
 
+        plan.ColumnGapWidths = Enumerable.Range(0, Math.Max(0, plan.Columns - 1))
+            .Select(i => i < plan.ColumnGapWidths.Count && double.IsFinite(plan.ColumnGapWidths[i])
+                ? Math.Clamp(plan.ColumnGapWidths[i], 0, 300)
+                : 0)
+            .ToList();
+
         var duplicateName = db.SeatingPlans.Find(p => p.GroupId == plan.GroupId)
             .FirstOrDefault(p => p.Id != plan.Id
                 && string.Equals(p.Name, plan.Name, StringComparison.OrdinalIgnoreCase)
@@ -191,15 +142,36 @@ public class SeatingPlanRepository(LiteDbContext db) : ISeatingPlanRepository
         if (plan.Assignments.Any(a => !memberIds.Contains(a.StudentId)))
             throw new InvalidOperationException("Der Sitzplan enthält einen Schüler, der nicht zur Lerngruppe gehört.");
 
+        // Ein ausgeblendeter Platz darf nie belegt sein - die Zuordnung eines Schülers ist
+        // wertvoller als das Ausblenden, deshalb wird hier stillschweigend wieder eingeblendet
+        // statt die Zuordnung zu verwerfen. Sollte über die UI (SeatingPlanTabViewModel verhindert
+        // das Belegen ausgeblendeter Plätze) nie vorkommen - reine Absicherung bei fehlerhaften
+        // oder von einem älteren Client synchronisierten Daten.
+        var assignedPositions = plan.Assignments.Select(a => (a.Row, a.Column)).ToHashSet();
+        plan.HiddenSeats = plan.HiddenSeats
+            .Where(h => h.Row >= 0 && h.Row < plan.Rows && h.Column >= 0 && h.Column < plan.Columns)
+            .Where(h => !assignedPositions.Contains((h.Row, h.Column)))
+            .DistinctBy(h => (h.Row, h.Column))
+            .ToList();
+
         plan.UpdatedAt = DateTime.UtcNow;
         db.SeatingPlans.Upsert(plan);
+        db.OnChange?.Invoke(nameof(SeatingPlan), plan.Id.ToString(), "Save", plan);
     }
 
     public void Delete(Guid id)
     {
-        if (db.SeatingPlans.FindById(id) is { } plan)
+        if (db.SeatingPlans.FindById(id) is not { } plan) return;
         ArchivedGroupWriteGuard.EnsureActive(db, plan.GroupId);
+        db.MoveToTrash(nameof(SeatingPlan), id, plan,
+            string.IsNullOrWhiteSpace(plan.Room) ? plan.Name : $"{plan.Name} (Raum {plan.Room})");
         db.SeatingPlans.Delete(id);
+        db.OnChange?.Invoke(nameof(SeatingPlan), id.ToString(), "Delete", null);
+    }
+
+    public void Restore(Guid trashId)
+    {
+        if (db.RestoreFromTrash<SeatingPlan>(trashId) is { } plan) Save(plan);
     }
 }
 
@@ -218,12 +190,14 @@ public class GroupMembershipRepository(LiteDbContext db) : IGroupMembershipRepos
         if (existing is not null && existing.Id != membership.Id)
             throw new InvalidOperationException("Der Schüler ist dieser Lerngruppe bereits zugeordnet.");
         db.Memberships.Upsert(membership);
+        db.OnChange?.Invoke(nameof(GroupMembership), membership.Id.ToString(), "Save", membership);
     }
     public void Delete(Guid id)
     {
         if (db.Memberships.FindById(id) is { } membership)
             ArchivedGroupWriteGuard.EnsureActive(db, membership.GroupId);
         db.Memberships.Delete(id);
+        db.OnChange?.Invoke(nameof(GroupMembership), id.ToString(), "Delete", null);
     }
 }
 
@@ -238,14 +212,14 @@ public class ExamRepository(LiteDbContext db) : IExamRepository
         ArchivedGroupWriteGuard.EnsureActive(db, e.GroupId);
         e.UpdatedAt = DateTime.UtcNow;
         db.Exams.Upsert(e);
+        db.OnChange?.Invoke(nameof(Exam), e.Id.ToString(), "Save", e);
     }
     public void Delete(Guid id)
     {
         if (db.Exams.FindById(id) is { } exam)
             ArchivedGroupWriteGuard.EnsureActive(db, exam.GroupId);
-        foreach (var result in db.ExamResults.Find(r => r.ExamId == id).ToList())
+        db.CascadeDeleteExam(id);
-            db.ExamResults.Delete(result.Id);
+        db.OnChange?.Invoke(nameof(Exam), id.ToString(), "Delete", null);
-        db.Exams.Delete(id);
     }
 }
 
@@ -263,6 +237,7 @@ public class ExamResultRepository(LiteDbContext db) : IExamResultRepository
             ArchivedGroupWriteGuard.EnsureActive(db, exam.GroupId);
         r.UpdatedAt = DateTime.UtcNow;
         db.ExamResults.Upsert(r);
+        db.OnChange?.Invoke(nameof(ExamResult), r.Id.ToString(), "Save", r);
     }
     public void SaveMany(List<ExamResult> results)
     {
@@ -272,6 +247,8 @@ public class ExamResultRepository(LiteDbContext db) : IExamResultRepository
         var now = DateTime.UtcNow;
         foreach (var r in results) r.UpdatedAt = now;
         db.ExamResults.Upsert(results);
+        foreach (var r in results)
+            db.OnChange?.Invoke(nameof(ExamResult), r.Id.ToString(), "Save", r);
     }
 }
 
@@ -282,8 +259,24 @@ public class GradingKeyTemplateRepository(LiteDbContext db) : IGradingKeyTemplat
     public List<GradingKeyTemplate> GetByGradingSystem(GradingSystem system) =>
         db.GradingKeyTemplates.Find(t => t.GradingSystem == system).OrderBy(t => t.Name).ToList();
     public GradingKeyTemplate? GetById(Guid id) => db.GradingKeyTemplates.FindById(id);
-    public void Save(GradingKeyTemplate t) { t.UpdatedAt = DateTime.UtcNow; db.GradingKeyTemplates.Upsert(t); }
+    public void Save(GradingKeyTemplate t)
-    public void Delete(Guid id) => db.GradingKeyTemplates.Delete(id);
+    {
+        t.UpdatedAt = DateTime.UtcNow;
+        db.GradingKeyTemplates.Upsert(t);
+        db.OnChange?.Invoke(nameof(GradingKeyTemplate), t.Id.ToString(), "Save", t);
+    }
+    public void Delete(Guid id)
+    {
+        if (db.GradingKeyTemplates.FindById(id) is { } template)
+            db.MoveToTrash(nameof(GradingKeyTemplate), id, template, template.Name);
+        db.GradingKeyTemplates.Delete(id);
+        db.OnChange?.Invoke(nameof(GradingKeyTemplate), id.ToString(), "Delete", null);
+    }
+
+    public void Restore(Guid trashId)
+    {
+        if (db.RestoreFromTrash<GradingKeyTemplate>(trashId) is { } template) Save(template);
+    }
 }
 
 public class GradeRepository(LiteDbContext db) : IGradeRepository
@@ -296,12 +289,21 @@ public class GradeRepository(LiteDbContext db) : IGradeRepository
     {
         ArchivedGroupWriteGuard.EnsureActive(db, g.GroupId);
         db.Grades.Upsert(g);
+        db.OnChange?.Invoke(nameof(Grade), g.Id.ToString(), "Save", g);
     }
     public void Delete(Guid id)
     {
-        if (db.Grades.FindById(id) is { } grade)
+        if (db.Grades.FindById(id) is not { } grade) return;
         ArchivedGroupWriteGuard.EnsureActive(db, grade.GroupId);
+        db.MoveToTrash(nameof(Grade), id, grade,
+            $"Note {grade.Value} ({grade.Date:dd.MM.yyyy})");
         db.Grades.Delete(id);
+        db.OnChange?.Invoke(nameof(Grade), id.ToString(), "Delete", null);
+    }
+
+    public void Restore(Guid trashId)
+    {
+        if (db.RestoreFromTrash<Grade>(trashId) is { } grade) Save(grade);
     }
 }
 
@@ -315,12 +317,14 @@ public class GradingSchemeRepository(LiteDbContext db) : IGradingSchemeRepositor
         if (s.GroupId is Guid groupId) ArchivedGroupWriteGuard.EnsureActive(db, groupId);
         s.UpdatedAt = DateTime.UtcNow;
         db.GradingSchemes.Upsert(s);
+        db.OnChange?.Invoke(nameof(GradingScheme), s.Id.ToString(), "Save", s);
     }
     public void Delete(Guid id)
     {
         if (db.GradingSchemes.FindById(id)?.GroupId is Guid groupId)
             ArchivedGroupWriteGuard.EnsureActive(db, groupId);
         db.GradingSchemes.Delete(id);
+        db.OnChange?.Invoke(nameof(GradingScheme), id.ToString(), "Delete", null);
     }
 }
 
@@ -334,12 +338,14 @@ public class ReportGradeRepository(LiteDbContext db) : IReportGradeRepository
         ArchivedGroupWriteGuard.EnsureActive(db, r.GroupId);
         r.UpdatedAt = DateTime.UtcNow;
         db.ReportGrades.Upsert(r);
+        db.OnChange?.Invoke(nameof(ReportGrade), r.Id.ToString(), "Save", r);
     }
     public void Delete(Guid id)
     {
         if (db.ReportGrades.FindById(id) is { } grade)
             ArchivedGroupWriteGuard.EnsureActive(db, grade.GroupId);
         db.ReportGrades.Delete(id);
+        db.OnChange?.Invoke(nameof(ReportGrade), id.ToString(), "Delete", null);
     }
 }
 
@@ -353,12 +359,14 @@ public class UnitRepository(LiteDbContext db) : IUnitRepository
         ArchivedGroupWriteGuard.EnsureActive(db, u.GroupId);
         u.UpdatedAt = DateTime.UtcNow;
         db.Units.Upsert(u);
+        db.OnChange?.Invoke(nameof(Unit), u.Id.ToString(), "Save", u);
     }
     public void Delete(Guid id)
     {
         if (db.Units.FindById(id) is { } unit)
             ArchivedGroupWriteGuard.EnsureActive(db, unit.GroupId);
         db.Units.Delete(id);
+        db.OnChange?.Invoke(nameof(Unit), id.ToString(), "Delete", null);
     }
 }
 
@@ -376,12 +384,17 @@ public class LessonRepository(LiteDbContext db) : ILessonRepository
         ArchivedGroupWriteGuard.EnsureActive(db, l.GroupId);
         l.UpdatedAt = DateTime.UtcNow;
         db.Lessons.Upsert(l);
+        db.OnChange?.Invoke(nameof(Lesson), l.Id.ToString(), "Save", l);
     }
     public void Delete(Guid id)
     {
         if (db.Lessons.FindById(id) is { } lesson)
+        {
             ArchivedGroupWriteGuard.EnsureActive(db, lesson.GroupId);
+            foreach (var attachment in lesson.Attachments) db.Attachments.Delete(attachment.StorageId);
+        }
         db.Lessons.Delete(id);
+        db.OnChange?.Invoke(nameof(Lesson), id.ToString(), "Delete", null);
     }
 }
 
@@ -394,7 +407,12 @@ public class DocumentationRepository(LiteDbContext db) : IDocumentationRepositor
             .OrderByDescending(d => d.Date).ToList();
     public List<Documentation> GetAll() =>
         db.Documentation.Find(d => !d.IsDeleted).OrderByDescending(d => d.Date).ToList();
-    public void Save(Documentation d) { d.UpdatedAt = DateTime.UtcNow; db.Documentation.Upsert(d); }
+    public void Save(Documentation d)
+    {
+        d.UpdatedAt = DateTime.UtcNow;
+        db.Documentation.Upsert(d);
+        db.OnChange?.Invoke(nameof(Documentation), d.Id.ToString(), "Save", d);
+    }
     public void Delete(Guid id)
     {
         var doc = db.Documentation.FindById(id);
@@ -402,13 +420,14 @@ public class DocumentationRepository(LiteDbContext db) : IDocumentationRepositor
         doc.IsDeleted = true;
         doc.DeletedAt = DateTime.UtcNow;
         db.Documentation.Update(doc);
+        // Weiches Löschen ist inhaltlich eine Änderung, kein Entfernen -> "Save", damit ein
+        // anwendendes Gerät den IsDeleted-Stand einfach übernimmt statt den Datensatz zu entfernen.
+        db.OnChange?.Invoke(nameof(Documentation), id.ToString(), "Save", doc);
     }
     public void HardDelete(Guid id)
     {
-        var doc = db.Documentation.FindById(id);
+        db.CascadeHardDeleteDocumentation(id);
-        if (doc is not null)
+        db.OnChange?.Invoke(nameof(Documentation), id.ToString(), "Delete", null);
-            foreach (var attachment in doc.Attachments) db.Attachments.Delete(attachment.StorageId);
-        db.Documentation.Delete(id);
     }
 }
 
@@ -418,8 +437,26 @@ public class WorkTaskRepository(LiteDbContext db) : IWorkTaskRepository
         db.Tasks.Find(t => t.Status == s).OrderBy(t => t.DueDate).ToList();
     public List<WorkTask> GetAll() =>
         db.Tasks.FindAll().OrderBy(t => t.Status).ThenBy(t => t.DueDate).ToList();
-    public void Save(WorkTask t) { t.UpdatedAt = DateTime.UtcNow; db.Tasks.Upsert(t); }
+    public List<WorkTask> GetByGroup(Guid groupId) =>
-    public void Delete(Guid id) => db.Tasks.Delete(id);
+        db.Tasks.Find(t => t.GroupId == groupId).OrderBy(t => t.Status).ThenBy(t => t.DueDate).ToList();
+    public void Save(WorkTask t)
+    {
+        t.UpdatedAt = DateTime.UtcNow;
+        db.Tasks.Upsert(t);
+        db.OnChange?.Invoke(nameof(WorkTask), t.Id.ToString(), "Save", t);
+    }
+    public void Delete(Guid id)
+    {
+        if (db.Tasks.FindById(id) is { } task)
+            db.MoveToTrash(nameof(WorkTask), id, task, task.Title);
+        db.Tasks.Delete(id);
+        db.OnChange?.Invoke(nameof(WorkTask), id.ToString(), "Delete", null);
+    }
+
+    public void Restore(Guid trashId)
+    {
+        if (db.RestoreFromTrash<WorkTask>(trashId) is { } task) Save(task);
+    }
 }
 
 public class TimeEntryRepository(LiteDbContext db) : ITimeEntryRepository
@@ -430,8 +467,24 @@ public class TimeEntryRepository(LiteDbContext db) : ITimeEntryRepository
         db.TimeEntries.Find(e => e.Date >= from && e.Date <= to).OrderBy(e => e.Date).ToList();
     public List<TimeEntry> GetByTask(Guid id) =>
         db.TimeEntries.Find(e => e.TaskId == id).ToList();
-    public void Save(TimeEntry e) => db.TimeEntries.Upsert(e);
+    public void Save(TimeEntry e)
-    public void Delete(Guid id) => db.TimeEntries.Delete(id);
+    {
+        db.TimeEntries.Upsert(e);
+        db.OnChange?.Invoke(nameof(TimeEntry), e.Id.ToString(), "Save", e);
+    }
+    public void Delete(Guid id)
+    {
+        if (db.TimeEntries.FindById(id) is { } entry)
+            db.MoveToTrash(nameof(TimeEntry), id, entry,
+                $"{entry.DurationMinutes} Min. {entry.Category} ({entry.Date:dd.MM.yyyy})");
+        db.TimeEntries.Delete(id);
+        db.OnChange?.Invoke(nameof(TimeEntry), id.ToString(), "Delete", null);
+    }
+
+    public void Restore(Guid trashId)
+    {
+        if (db.RestoreFromTrash<TimeEntry>(trashId) is { } entry) Save(entry);
+    }
 }
 
 public class ParticipationSessionRepository(LiteDbContext db) : IParticipationSessionRepository
@@ -444,14 +497,14 @@ public class ParticipationSessionRepository(LiteDbContext db) : IParticipationSe
         ArchivedGroupWriteGuard.EnsureActive(db, s.GroupId);
         s.UpdatedAt = DateTime.UtcNow;
         db.ParticipationSessions.Upsert(s);
+        db.OnChange?.Invoke(nameof(ParticipationSession), s.Id.ToString(), "Save", s);
     }
     public void Delete(Guid id)
     {
         if (db.ParticipationSessions.FindById(id) is { } session)
             ArchivedGroupWriteGuard.EnsureActive(db, session.GroupId);
-        db.ParticipationSessions.Delete(id);
+        db.CascadeDeleteParticipationSession(id);
-        foreach (var e in db.ParticipationEntries.Find(e => e.SessionId == id).ToList())
+        db.OnChange?.Invoke(nameof(ParticipationSession), id.ToString(), "Delete", null);
-            db.ParticipationEntries.Delete(e.Id);
     }
 }
 
@@ -469,6 +522,7 @@ public class ParticipationRepository(LiteDbContext db) : IParticipationRepositor
             ArchivedGroupWriteGuard.EnsureActive(db, session.GroupId);
         e.UpdatedAt = DateTime.UtcNow;
         db.ParticipationEntries.Upsert(e);
+        db.OnChange?.Invoke(nameof(ParticipationEntry), e.Id.ToString(), "Save", e);
     }
     public void SaveMany(List<ParticipationEntry> entries)
     {
@@ -478,13 +532,18 @@ public class ParticipationRepository(LiteDbContext db) : IParticipationRepositor
         var now = DateTime.UtcNow;
         foreach (var e in entries) e.UpdatedAt = now;
         db.ParticipationEntries.Upsert(entries);
+        foreach (var e in entries)
+            db.OnChange?.Invoke(nameof(ParticipationEntry), e.Id.ToString(), "Save", e);
     }
     public void DeleteBySession(Guid sessionId)
     {
         if (db.ParticipationSessions.FindById(sessionId) is { } session)
             ArchivedGroupWriteGuard.EnsureActive(db, session.GroupId);
         foreach (var e in db.ParticipationEntries.Find(e => e.SessionId == sessionId).ToList())
+        {
             db.ParticipationEntries.Delete(e.Id);
+            db.OnChange?.Invoke(nameof(ParticipationEntry), e.Id.ToString(), "Delete", null);
+        }
     }
 }
 
@@ -514,12 +573,14 @@ public class ParticipationAspectRepository(LiteDbContext db) : IParticipationAsp
             throw new InvalidOperationException("Ein Aspekt mit diesem Schlüssel existiert für diese Gruppe bereits.");
         a.UpdatedAt = DateTime.UtcNow;
         db.ParticipationAspects.Upsert(a);
+        db.OnChange?.Invoke(nameof(ParticipationAspect), a.Id.ToString(), "Save", a);
     }
     public void Delete(Guid id)
     {
         if (db.ParticipationAspects.FindById(id)?.GroupId is Guid groupId)
             ArchivedGroupWriteGuard.EnsureActive(db, groupId);
         db.ParticipationAspects.Delete(id);
+        db.OnChange?.Invoke(nameof(ParticipationAspect), id.ToString(), "Delete", null);
     }
 }
 
@@ -531,12 +592,14 @@ public class ParticipationSectionRepository(LiteDbContext db) : IParticipationSe
     {
         ArchivedGroupWriteGuard.EnsureActive(db, s.GroupId);
         db.ParticipationSections.Upsert(s);
+        db.OnChange?.Invoke(nameof(ParticipationSection), s.Id.ToString(), "Save", s);
     }
     public void Delete(Guid id)
     {
         if (db.ParticipationSections.FindById(id) is { } section)
             ArchivedGroupWriteGuard.EnsureActive(db, section.GroupId);
         db.ParticipationSections.Delete(id);
+        db.OnChange?.Invoke(nameof(ParticipationSection), id.ToString(), "Delete", null);
     }
 }
 
@@ -556,12 +619,14 @@ public class SubjectRepository(LiteDbContext db) : ISubjectRepository
             throw new InvalidOperationException("Ein Fach mit diesem Namen existiert bereits.");
         s.UpdatedAt = DateTime.UtcNow;
         db.Subjects.Upsert(s);
+        db.OnChange?.Invoke(nameof(Subject), s.Id.ToString(), "Save", s);
     }
     public void Delete(Guid id)
     {
         if (db.Groups.Exists(g => g.SubjectId == id) || db.CompetencyDomains.Exists(d => d.SubjectId == id))
             throw new InvalidOperationException("Das Fach wird noch von einer Lerngruppe oder einem Kompetenzkatalog verwendet.");
         db.Subjects.Delete(id);
+        db.OnChange?.Invoke(nameof(Subject), id.ToString(), "Delete", null);
     }
 }
 
@@ -579,8 +644,13 @@ public class ShorthandCodeRepository(LiteDbContext db) : IShorthandCodeRepositor
             throw new InvalidOperationException("Ein Kürzel mit diesem Code existiert bereits.");
         c.UpdatedAt = DateTime.UtcNow;
         db.ShorthandCodes.Upsert(c);
+        db.OnChange?.Invoke(nameof(ShorthandCode), c.Id.ToString(), "Save", c);
+    }
+    public void Delete(Guid id)
+    {
+        db.ShorthandCodes.Delete(id);
+        db.OnChange?.Invoke(nameof(ShorthandCode), id.ToString(), "Delete", null);
     }
-    public void Delete(Guid id) => db.ShorthandCodes.Delete(id);
 }
 
 public class AlternativeLessonPathRepository(LiteDbContext db) : IAlternativeLessonPathRepository
@@ -598,8 +668,13 @@ public class AlternativeLessonPathRepository(LiteDbContext db) : IAlternativeLes
             throw new InvalidOperationException("Ein alternativer Ablauf mit diesem Namen existiert bereits.");
         p.UpdatedAt = DateTime.UtcNow;
         db.AlternativeLessonPaths.Upsert(p);
+        db.OnChange?.Invoke(nameof(AlternativeLessonPath), p.Id.ToString(), "Save", p);
+    }
+    public void Delete(Guid id)
+    {
+        db.AlternativeLessonPaths.Delete(id);
+        db.OnChange?.Invoke(nameof(AlternativeLessonPath), id.ToString(), "Delete", null);
     }
-    public void Delete(Guid id) => db.AlternativeLessonPaths.Delete(id);
 }
 
 public class TimetableSlotRepository(LiteDbContext db) : ITimetableSlotRepository
@@ -616,20 +691,30 @@ public class TimetableSlotRepository(LiteDbContext db) : ITimetableSlotRepositor
         if (occupied is not null && occupied.Id != slot.Id)
             throw new InvalidOperationException("Diese Stunde ist bereits belegt.");
         db.TimetableSlots.Upsert(slot);
+        db.OnChange?.Invoke(nameof(TimetableSlot), slot.Id.ToString(), "Save", slot);
     }
     public void Delete(Guid id)
     {
         if (db.TimetableSlots.FindById(id) is { } slot)
             ArchivedGroupWriteGuard.EnsureActive(db, slot.GroupId);
         db.TimetableSlots.Delete(id);
+        db.OnChange?.Invoke(nameof(TimetableSlot), id.ToString(), "Delete", null);
     }
 }
 
 public class SchoolHolidayRepository(LiteDbContext db) : ISchoolHolidayRepository
 {
     public List<SchoolHoliday> GetAll() => db.SchoolHolidays.FindAll().OrderBy(h => h.StartDate).ToList();
-    public void Save(SchoolHoliday holiday) => db.SchoolHolidays.Upsert(holiday);
+    public void Save(SchoolHoliday holiday)
-    public void Delete(Guid id) => db.SchoolHolidays.Delete(id);
+    {
+        db.SchoolHolidays.Upsert(holiday);
+        db.OnChange?.Invoke(nameof(SchoolHoliday), holiday.Id.ToString(), "Save", holiday);
+    }
+    public void Delete(Guid id)
+    {
+        db.SchoolHolidays.Delete(id);
+        db.OnChange?.Invoke(nameof(SchoolHoliday), id.ToString(), "Delete", null);
+    }
 }
 
 public class SupervisionDutyRepository(LiteDbContext db) : ISupervisionDutyRepository
@@ -643,8 +728,13 @@ public class SupervisionDutyRepository(LiteDbContext db) : ISupervisionDutyRepos
         if (occupied is not null && occupied.Id != duty.Id)
             throw new InvalidOperationException("Für diese Pause ist bereits eine Aufsicht eingetragen.");
         db.SupervisionDuties.Upsert(duty);
+        db.OnChange?.Invoke(nameof(SupervisionDuty), duty.Id.ToString(), "Save", duty);
+    }
+    public void Delete(Guid id)
+    {
+        db.SupervisionDuties.Delete(id);
+        db.OnChange?.Invoke(nameof(SupervisionDuty), id.ToString(), "Delete", null);
     }
-    public void Delete(Guid id) => db.SupervisionDuties.Delete(id);
 }
 
 public class SubstitutionEntryRepository(LiteDbContext db) : ISubstitutionEntryRepository
@@ -652,8 +742,115 @@ public class SubstitutionEntryRepository(LiteDbContext db) : ISubstitutionEntryR
     public List<SubstitutionEntry> GetAll() => db.SubstitutionEntries.FindAll().OrderBy(e => e.Date).ToList();
     public List<SubstitutionEntry> GetByDate(DateOnly date) =>
         db.SubstitutionEntries.Find(e => e.Date == date).ToList();
-    public void Save(SubstitutionEntry entry) => db.SubstitutionEntries.Upsert(entry);
+    public SubstitutionEntry? GetByExternalId(string externalId) =>
-    public void Delete(Guid id) => db.SubstitutionEntries.Delete(id);
+        db.SubstitutionEntries.FindOne(e => e.ExternalId == externalId);
+    public void Save(SubstitutionEntry entry)
+    {
+        db.SubstitutionEntries.Upsert(entry);
+        db.OnChange?.Invoke(nameof(SubstitutionEntry), entry.Id.ToString(), "Save", entry);
+    }
+    public void Delete(Guid id)
+    {
+        db.SubstitutionEntries.Delete(id);
+        db.OnChange?.Invoke(nameof(SubstitutionEntry), id.ToString(), "Delete", null);
+    }
+}
+
+// Bewusst kein db.OnChange hier (anders als sonst überall): UntisSnapshotEntry ist eine rein
+// lokale, hochfrequente Abgleich-Zwischenablage (jeder Poll aktualisiert alle Zeilen), deren
+// Sync nur Rauschen erzeugen würde. UntisSlotMapping ist an die pro Gerät hinterlegte
+// WebUntis-URL gebunden (WebUntisSettingsService, wie SyncSettingsService nicht synchronisiert)
+// und deshalb ebenfalls sinnvollerweise lokal — jedes Gerät bestätigt seine Zuordnung einmal
+// selbst über den Review-Dialog.
+public class UntisSnapshotRepository(LiteDbContext db) : IUntisSnapshotRepository
+{
+    public List<UntisSnapshotEntry> GetAll() => db.UntisSnapshotEntries.FindAll().ToList();
+    public void Save(UntisSnapshotEntry entry) => db.UntisSnapshotEntries.Upsert(entry);
+    public void Delete(Guid id) => db.UntisSnapshotEntries.Delete(id);
+}
+
+public class UntisSlotMappingRepository(LiteDbContext db) : IUntisSlotMappingRepository
+{
+    public List<UntisSlotMapping> GetAll() => db.UntisSlotMappings.FindAll().ToList();
+    public void Save(UntisSlotMapping mapping) => db.UntisSlotMappings.Upsert(mapping);
+    public void Delete(Guid id) => db.UntisSlotMappings.Delete(id);
+}
+
+// Wie UntisSnapshotEntry rein lokal: Der Jahresplan ist ein wiederherstellbarer Cache eines
+// externen Feeds. db.OnChange würde bei jedem vollständigen Abruf hunderte Sync-Ereignisse erzeugen.
+public class AnnualPlanEventRepository(LiteDbContext db) : IAnnualPlanEventRepository
+{
+    public List<AnnualPlanEvent> GetAll() =>
+        db.AnnualPlanEvents.FindAll().OrderBy(e => e.StartDate).ThenBy(e => e.StartTime).ToList();
+
+    public List<AnnualPlanEvent> GetByRange(DateOnly from, DateOnly to) =>
+        db.AnnualPlanEvents.Find(e => e.StartDate <= to && e.EndDate >= from)
+            .OrderBy(e => e.StartDate).ThenBy(e => e.StartTime).ToList();
+
+    public AnnualPlanEvent? GetByExternalId(string externalId) =>
+        db.AnnualPlanEvents.FindOne(e => e.ExternalId == externalId);
+
+    public void Save(AnnualPlanEvent entry) => db.AnnualPlanEvents.Upsert(entry);
+    public void Delete(Guid id) => db.AnnualPlanEvents.Delete(id);
+}
+
+// Wie UntisSnapshotEntry/AnnualPlanEvent rein lokal, bewusst kein db.OnChange: Fehlzeiten- und
+// Klassenbuch-Cache fürs Klassenlehrer-Feature (siehe TODO.md) wachsen über ein Schuljahr auf viele
+// hundert Zeilen an - über Sync würde das nur Rauschen erzeugen, und jedes Gerät ruft WebUntis
+// ohnehin selbst ab (siehe UntisReportCacheService).
+public class UntisAbsenceCacheRepository(LiteDbContext db) : IUntisAbsenceCacheRepository
+{
+    public List<UntisAbsenceCacheEntry> GetByClassAndRange(string className, int startDate, int endDate) =>
+        db.UntisAbsenceCache
+          .Find(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate)
+          .ToList();
+
+    public void ReplaceRange(string className, int startDate, int endDate, IEnumerable<UntisAbsenceCacheEntry> entries)
+    {
+        db.UntisAbsenceCache.DeleteMany(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate);
+        db.UntisAbsenceCache.InsertBulk(entries);
+    }
+
+    public void InsertRange(IEnumerable<UntisAbsenceCacheEntry> entries) => db.UntisAbsenceCache.InsertBulk(entries);
+}
+
+public class UntisClassRegisterCacheRepository(LiteDbContext db) : IUntisClassRegisterCacheRepository
+{
+    public List<UntisClassRegisterCacheEntry> GetByClassAndRange(string className, int startDate, int endDate) =>
+        db.UntisClassRegisterCache
+          .Find(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate)
+          .ToList();
+
+    public void ReplaceRange(string className, int startDate, int endDate, IEnumerable<UntisClassRegisterCacheEntry> entries)
+    {
+        db.UntisClassRegisterCache.DeleteMany(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate);
+        db.UntisClassRegisterCache.InsertBulk(entries);
+    }
+
+    public void InsertRange(IEnumerable<UntisClassRegisterCacheEntry> entries) => db.UntisClassRegisterCache.InsertBulk(entries);
+}
+
+// Gleiches "bewusst kein db.OnChange"-Prinzip wie UntisAbsenceCacheRepository oben, auch wenn der
+// Roster selbst klein bleibt (aktuelle Klassenliste, keine Historie) - jedes Gerät ruft ihn ohnehin
+// selbst ab.
+public class UntisStudentRosterCacheRepository(LiteDbContext db) : IUntisStudentRosterCacheRepository
+{
+    public List<UntisStudentRosterCacheEntry> GetByClass(string className) =>
+        db.UntisStudentRosterCache.Find(e => e.ClassName == className).ToList();
+
+    public void ReplaceAll(string className, IEnumerable<UntisStudentRosterCacheEntry> entries)
+    {
+        db.UntisStudentRosterCache.DeleteMany(e => e.ClassName == className);
+        db.UntisStudentRosterCache.InsertBulk(entries);
+    }
+}
+
+public class UntisCacheFetchStateRepository(LiteDbContext db) : IUntisCacheFetchStateRepository
+{
+    public UntisCacheFetchState? Get(string className, UntisCacheKind kind) =>
+        db.UntisCacheFetchStates.FindOne(s => s.ClassName == className && s.Kind == kind);
+
+    public void Save(UntisCacheFetchState state) => db.UntisCacheFetchStates.Upsert(state);
 }
 
 public class CompetencyDomainRepository(LiteDbContext db) : ICompetencyDomainRepository
@@ -664,14 +861,26 @@ public class CompetencyDomainRepository(LiteDbContext db) : ICompetencyDomainRep
           .OrderBy(d => d.SortOrder)
           .ToList();
     public CompetencyDomain? GetById(Guid id) => db.CompetencyDomains.FindById(id);
-    public void Save(CompetencyDomain d) { d.UpdatedAt = DateTime.UtcNow; db.CompetencyDomains.Upsert(d); }
+    public void Save(CompetencyDomain d)
-    public void Delete(Guid id) => db.CompetencyDomains.Delete(id);
+    {
+        d.UpdatedAt = DateTime.UtcNow;
+        db.CompetencyDomains.Upsert(d);
+        db.OnChange?.Invoke(nameof(CompetencyDomain), d.Id.ToString(), "Save", d);
+    }
+    public void Delete(Guid id)
+    {
+        db.CompetencyDomains.Delete(id);
+        db.OnChange?.Invoke(nameof(CompetencyDomain), id.ToString(), "Delete", null);
+    }
     public void DeleteBySubjectAndGrade(Guid subjectId, int gradeLevel)
     {
         foreach (var d in db.CompetencyDomains
                              .Find(d => d.SubjectId == subjectId && d.GradeLevel == gradeLevel)
                              .ToList())
+        {
             db.CompetencyDomains.Delete(d.Id);
+            db.OnChange?.Invoke(nameof(CompetencyDomain), d.Id.ToString(), "Delete", null);
+        }
     }
     public void ReplaceForSubjectAndGrade(Guid subjectId, int gradeLevel, List<CompetencyDomain> domains)
     {
@@ -679,7 +888,10 @@ public class CompetencyDomainRepository(LiteDbContext db) : ICompetencyDomainRep
         {
             foreach (var domain in db.CompetencyDomains
                          .Find(d => d.SubjectId == subjectId && d.GradeLevel == gradeLevel).ToList())
+            {
                 db.CompetencyDomains.Delete(domain.Id);
+                db.OnChange?.Invoke(nameof(CompetencyDomain), domain.Id.ToString(), "Delete", null);
+            }
             var now = DateTime.UtcNow;
             foreach (var domain in domains)
             {
@@ -687,7 +899,23 @@ public class CompetencyDomainRepository(LiteDbContext db) : ICompetencyDomainRep
                 domain.GradeLevel = gradeLevel;
                 domain.UpdatedAt = now;
                 db.CompetencyDomains.Upsert(domain);
+                db.OnChange?.Invoke(nameof(CompetencyDomain), domain.Id.ToString(), "Save", domain);
             }
         });
     }
 }
+
+/// <summary>Papierkorb (14.3) — bewusst nicht sync-fähig: TrashedItem wird nie über
+/// db.OnChange gemeldet, bleibt also rein lokal auf dem Gerät, auf dem gelöscht wurde. Ein
+/// "Fehlklick sofort rückgängig machen"-Werkzeug, kein geräteübergreifendes Archiv.</summary>
+public class TrashRepository(LiteDbContext db) : ITrashRepository
+{
+    public List<TrashedItem> GetAll() =>
+        db.TrashedItems.FindAll().OrderByDescending(t => t.DeletedAt).ToList();
+
+    public void PurgeOlderThan(DateTime cutoffUtc)
+    {
+        foreach (var item in db.TrashedItems.Find(t => t.DeletedAt < cutoffUtc).ToList())
+            db.TrashedItems.Delete(item.Id);
+    }
+}

LehrerApp.Desktop.Tests/AnnualPlanSettingsServiceTests.cs

@@ -0,0 +1,46 @@
+using LehrerApp.Desktop.Services;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class AnnualPlanSettingsServiceTests
+{
+    private const string SampleUrl = "https://example.org/export.php?type=ics&token=intern";
+
+    private static string TempPath()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"lehrerapp-annualplansettingssvc-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(path);
+        return path;
+    }
+
+    [Fact]
+    public void IcalUrl_WirdVerschluesseltPersistiertUndWiederGeladen()
+    {
+        var path = TempPath();
+        var service = new AnnualPlanSettingsService(path);
+        service.SetIcalUrl(SampleUrl);
+        service.SetEnabled(true);
+
+        var raw = File.ReadAllText(Path.Combine(path, "annual-plan-settings.json"));
+        var reloaded = new AnnualPlanSettingsService(path);
+
+        Assert.DoesNotContain("token=intern", raw);
+        Assert.Equal(SampleUrl, reloaded.GetIcalUrl());
+        Assert.True(reloaded.Enabled);
+    }
+
+    [Fact]
+    public void ClearIcalUrl_EntferntUrlUndDeaktiviertDenImport()
+    {
+        var service = new AnnualPlanSettingsService(TempPath());
+        service.SetIcalUrl(SampleUrl);
+        service.SetEnabled(true);
+
+        service.ClearIcalUrl();
+
+        Assert.False(service.IsConfigured);
+        Assert.False(service.Enabled);
+        Assert.Null(service.GetIcalUrl());
+    }
+}

LehrerApp.Desktop.Tests/AnnualPlanSyncServiceTests.cs

@@ -0,0 +1,73 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.Services;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class AnnualPlanSyncServiceTests
+{
+    private static string Calendar(params string[] events) =>
+        "BEGIN:VCALENDAR\nVERSION:2.0\n" + string.Join("", events) + "END:VCALENDAR\n";
+
+    private static string Event(string uid, string title, string date = "20260823") =>
+        $"BEGIN:VEVENT\nUID:{uid}\nDTSTART;VALUE=DATE:{date}\nSUMMARY:{title}\nEND:VEVENT\n";
+
+    private static AnnualPlanSyncService Build(FakeAnnualPlanEvents events) =>
+        new(new HttpClient(), TestSupport.BuildAnnualPlanSettingsService(), events);
+
+    [Fact]
+    public void ProcessIcsText_LegtAn_AktualisiertIdempotent_UndEntferntVerschwundeneTermine()
+    {
+        var events = new FakeAnnualPlanEvents();
+        using var service = Build(events);
+
+        var first = service.ProcessIcsText(Calendar(Event("1", "Konferenz"), Event("2", "Messe")));
+        var originalId = events.GetByExternalId("1")!.Id;
+        var unchanged = service.ProcessIcsText(Calendar(Event("1", "Konferenz"), Event("2", "Messe")));
+        var changed = service.ProcessIcsText(Calendar(Event("1", "Konferenz verschoben", "20260824")));
+
+        Assert.Equal((2, 0, 0), (first.Added, first.Updated, first.Deleted));
+        Assert.Equal(0, unchanged.ChangeCount);
+        Assert.Equal((0, 1, 1), (changed.Added, changed.Updated, changed.Deleted));
+        Assert.Equal(originalId, events.GetByExternalId("1")!.Id);
+        Assert.Null(events.GetByExternalId("2"));
+    }
+
+    [Fact]
+    public void ProcessIcsText_DefekterTeilimport_VeraendertBestehendenBestandNicht()
+    {
+        var events = new FakeAnnualPlanEvents();
+        events.Add(new AnnualPlanEvent
+        {
+            ExternalId = "bestand", Title = "Bestehend",
+            StartDate = new DateOnly(2026, 8, 23), EndDate = new DateOnly(2026, 8, 23), IsAllDay = true,
+        });
+        using var service = Build(events);
+        var broken = Calendar(Event("neu", "Neu"),
+            "BEGIN:VEVENT\nUID:defekt\nSUMMARY:Ohne Datum\nEND:VEVENT\n");
+
+        Assert.Throws<FormatException>(() => service.ProcessIcsText(broken));
+
+        var remaining = Assert.Single(events.GetAll());
+        Assert.Equal("bestand", remaining.ExternalId);
+    }
+
+    [Fact]
+    public void Clear_EntferntDenLokalenFeedCache()
+    {
+        var events = new FakeAnnualPlanEvents();
+        events.Add(new AnnualPlanEvent
+        {
+            ExternalId = "1", StartDate = new DateOnly(2026, 8, 23),
+            EndDate = new DateOnly(2026, 8, 23), IsAllDay = true,
+        });
+        using var service = Build(events);
+        var notified = false;
+        service.DataChanged += () => notified = true;
+
+        service.Clear();
+
+        Assert.Empty(events.GetAll());
+        Assert.True(notified);
+    }
+}

LehrerApp.Desktop.Tests/AppearanceSettingsServiceTests.cs

@@ -0,0 +1,62 @@
+using LehrerApp.Desktop.Services;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class AppearanceSettingsServiceTests
+{
+    private static string BuildTempPath()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"lehrerapp-appearance-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(path);
+        return path;
+    }
+
+    [Fact]
+    public void Load_OhneVorhandeneDatei_LiefertSystemvorgabe() =>
+        Assert.Equal(AppTheme.System, new AppearanceSettingsService(BuildTempPath()).Load());
+
+    [Theory]
+    [InlineData(AppTheme.Light)]
+    [InlineData(AppTheme.Dark)]
+    [InlineData(AppTheme.System)]
+    public void SaveUndLoad_PersistiertUeberNeueInstanz(AppTheme theme)
+    {
+        var path = BuildTempPath();
+        new AppearanceSettingsService(path).Save(theme);
+
+        var reloaded = new AppearanceSettingsService(path).Load();
+
+        Assert.Equal(theme, reloaded);
+    }
+
+    [Fact]
+    public void Load_BeschaedigteDatei_FaelltAufSystemvorgabeZurueck()
+    {
+        var path = BuildTempPath();
+        File.WriteAllText(Path.Combine(path, "appearancesettings.json"), "{ kein json");
+
+        Assert.Equal(AppTheme.System, new AppearanceSettingsService(path).Load());
+    }
+}
+
+public sealed class AppThemeDisplayTests
+{
+    [Theory]
+    [InlineData(AppTheme.System, "Systemvorgabe")]
+    [InlineData(AppTheme.Light, "Hell")]
+    [InlineData(AppTheme.Dark, "Dunkel")]
+    public void Label_LiefertDeutscheBeschriftung(AppTheme theme, string expected) =>
+        Assert.Equal(expected, AppThemeDisplay.Label(theme));
+
+    [Fact]
+    public void FromLabel_UndLabel_SindZueinanderInvers()
+    {
+        foreach (var theme in Enum.GetValues<AppTheme>())
+            Assert.Equal(theme, AppThemeDisplay.FromLabel(AppThemeDisplay.Label(theme)));
+    }
+
+    [Fact]
+    public void FromLabel_UnbekanntesLabel_LiefertSystemvorgabe() =>
+        Assert.Equal(AppTheme.System, AppThemeDisplay.FromLabel("Nonsens"));
+}

LehrerApp.Desktop.Tests/AssemblyInfo.cs

@@ -0,0 +1,7 @@
+using Xunit;
+
+// Gleicher Grund wie in LehrerApp.Data.Tests/AssemblyInfo.cs: LiteDBs geteilter, statischer
+// BsonMapper.Global verträgt keine parallele Erstzuordnung von Typ-Metadaten über mehrere
+// Testklassen hinweg (SettingsViewModelTests/SyncStatusViewModelTests konstruieren beide
+// LiteDbContext).
+[assembly: CollectionBehavior(DisableTestParallelization = true)]

LehrerApp.Desktop.Tests/ClassTeacherViewModelsTests.cs

@@ -0,0 +1,412 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.Services;
+using LehrerApp.Desktop.ViewModels.ClassTeacher;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class ClassTeacherViewModelsTests
+{
+    [Fact]
+    public void GroupByStudentAndDay_FasstFehlstundenProSchuelerUndTagZusammen()
+    {
+        var entries = new[]
+        {
+            new UntisClassAbsenceEntryDto("Muster Max", 1, "6a", 20260824, 1, 45, "HED", "Che", "Absent",
+                "Krank", 1, "24.08.26", true, null, 3, "nicht entsch.", false),
+            new UntisClassAbsenceEntryDto("Muster Max", 1, "6a", 20260824, 1, 45, "PAN", "Kunst", null,
+                "Krank", 2, "24.08.26", true, null, 5, "nicht entsch.", true),
+            new UntisClassAbsenceEntryDto("Andere Schuelerin", 2, "6a", 20260824, 1, 20, "BEN", "Deu",
+                "Verspätung", null, 3, "24.08.26", false, "Zug verpasst", 1, "entsch.", false),
+        };
+
+        var rows = ClassAbsenceDaySummaryRow.GroupByStudentAndDay(entries);
+
+        Assert.Equal(2, rows.Count);
+
+        var max = rows[1];
+        Assert.Equal("Muster Max", max.StudentName);
+        Assert.Equal(2, max.TotalAbsentPeriods);
+        Assert.Equal(90, max.TotalAbsentMinutes);
+        Assert.Equal(["Che", "Kunst"], max.Subjects);
+        Assert.Equal([3, 5], max.PeriodNumbers.Order());
+        Assert.Equal(["nicht entsch."], max.Statuses);
+        Assert.Equal(["Absent"], max.AbsenceReasons);
+        Assert.Equal("Krank", max.Note);
+        Assert.Null(max.ExcuseNote);
+        Assert.True(max.CountsAsFullDay);
+
+        var andere = rows[0];
+        Assert.Equal("Andere Schuelerin", andere.StudentName);
+        Assert.Equal(1, andere.TotalAbsentPeriods);
+        Assert.Equal(20, andere.TotalAbsentMinutes);
+        Assert.Equal(["Deu"], andere.Subjects);
+        Assert.Null(andere.Note);
+        Assert.Equal("Zug verpasst", andere.ExcuseNote);
+        Assert.False(andere.CountsAsFullDay);
+    }
+
+    [Fact]
+    public void GroupByStudentAndDay_TrenntGleichenNamenAnUnterschiedlichenTagen()
+    {
+        var entries = new[]
+        {
+            new UntisClassAbsenceEntryDto("Muster Max", 1, "6a", 20260824, 1, 45, "HED", "Che", null,
+                null, 1, "24.08.26", true, null, 3, "nicht entsch.", false),
+            new UntisClassAbsenceEntryDto("Muster Max", 1, "6a", 20260825, 1, 45, "HED", "Che", null,
+                null, 2, "25.08.26", true, null, 3, "nicht entsch.", false),
+        };
+
+        var rows = ClassAbsenceDaySummaryRow.GroupByStudentAndDay(entries);
+
+        Assert.Equal(2, rows.Count);
+        Assert.Equal(new DateOnly(2026, 8, 25), rows[0].Date);
+        Assert.Equal(new DateOnly(2026, 8, 24), rows[1].Date);
+    }
+
+    [Fact]
+    public void RosterBuild_OrdnetFehlzeitPerExternKeyUndKlassenbuchPerNameZu()
+    {
+        var students = new[]
+        {
+            Student(1001, "Ada Müller"),
+            Student(1002, "Ben Schmidt"),
+            Student(1003, "Cem Yilmaz"),
+        };
+        var todayAbsences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 25), "Ada Müller", 1001, 2, 90,
+                ["Che"], [3, 4], ["nicht entsch."], ["Absent"], null, null, false),
+        };
+        var recentClassRegister = new[]
+        {
+            new UntisForeignClassRegisterEventDto("6a", 20260824, "Deu", "Ben Schmidt", "mueller",
+                "Vergessen", "Organisation", "Buch vergessen"),
+        };
+
+        var roster = ClassTeacherRosterRow.Build(students, todayAbsences, recentClassRegister);
+
+        Assert.Equal(3, roster.Count);
+        var ada = Assert.Single(roster, r => r.StudentName == "Ada Müller");
+        Assert.True(ada.HasAbsenceToday);
+        Assert.Contains("nicht entsch.", ada.AbsenceTooltip);
+        Assert.False(ada.HasRecentClassRegisterEntry);
+
+        var ben = Assert.Single(roster, r => r.StudentName == "Ben Schmidt");
+        Assert.False(ben.HasAbsenceToday);
+        Assert.True(ben.HasRecentClassRegisterEntry);
+
+        var cem = Assert.Single(roster, r => r.StudentName == "Cem Yilmaz");
+        Assert.False(cem.HasAbsenceToday);
+        Assert.False(cem.HasRecentClassRegisterEntry);
+    }
+
+    [Fact]
+    public void RosterBuild_ErkenntNamenAuchInVertauschterReihenfolge()
+    {
+        // WebUntis liefert Namen im Schülerreport als "Vorname Nachname", in Fehlzeiten-/
+        // Klassenbuchberichten aber typischerweise als "Nachname Vorname" - der Namensabgleich
+        // muss das auch ohne ExternKey erkennen (Regressionstest für genau diesen Bug).
+        var students = new[] { Student(externKey: null, displayName: "Ben Schmidt") };
+        var todayAbsences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 25), "Schmidt Ben", null, 1, 45,
+                ["Che"], [3], ["nicht entsch."], ["Absent"], null, null, false),
+        };
+        var recentClassRegister = new[]
+        {
+            new UntisForeignClassRegisterEventDto("6a", 20260824, "Deu", "Schmidt Ben", "mueller",
+                "Vergessen", "Organisation", "Buch vergessen"),
+        };
+
+        var roster = ClassTeacherRosterRow.Build(students, todayAbsences, recentClassRegister);
+
+        var ben = Assert.Single(roster);
+        Assert.True(ben.HasAbsenceToday);
+        Assert.True(ben.HasRecentClassRegisterEntry);
+    }
+
+    [Fact]
+    public void RosterBuild_BeschreibtVerspaetungNichtAlsNullFehlstunden()
+    {
+        var students = new[] { Student(1001, "Ada Müller") };
+        var todayAbsences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 26), "Müller Ada", 1001, 0, 20,
+                ["Deu"], [1], ["nicht entsch."], ["Verspätung"], null, null, false),
+        };
+
+        var row = Assert.Single(ClassTeacherRosterRow.Build(students, todayAbsences, [],
+            new DateOnly(2026, 8, 26)));
+
+        Assert.True(row.IsLate);
+        Assert.True(row.IsUnexcused);
+        Assert.Equal("20 Min. verspätet · Unentschuldigt", row.StatusText);
+        Assert.Equal(ClassTeacherStatusKind.Danger, row.StatusKind);
+    }
+
+    [Fact]
+    public void RosterBuild_PriorisiertHandlungsbedarfVorUnauffaelligenSchuelern()
+    {
+        var students = new[]
+        {
+            Student(1001, "Zora Unauffällig"),
+            Student(1002, "Ada Fehlzeit"),
+            Student(1003, "Ben Klassenbuch"),
+        };
+        var absences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 26), "Fehlzeit Ada", 1002, 2, 90,
+                ["Deu"], [1, 2], ["nicht entsch."], ["Absent"], null, null, false),
+        };
+        var register = new[]
+        {
+            new UntisForeignClassRegisterEventDto("10c", 20260825, "Deu", "Klassenbuch Ben", "test",
+                "Fehlende HA", "Negativ", "Hausaufgaben fehlen"),
+        };
+
+        var rows = ClassTeacherRosterRow.Build(students, absences, register, new DateOnly(2026, 8, 26));
+
+        Assert.Equal(["Ada Fehlzeit", "Ben Klassenbuch", "Zora Unauffällig"],
+            rows.Select(r => r.StudentName));
+    }
+
+    [Fact]
+    public void RosterBuild_BerechnetFehlquoteSeitSchuljahresbeginn()
+    {
+        var students = new[] { Student(1001, "Ada Müller") };
+        var yearAbsences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 10), "Müller Ada", 1001, 2, 90,
+                ["Che"], [1, 2], ["entsch."], ["Krank"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 17), "Müller Ada", 1001, 6, 270,
+                ["Che"], [1, 2, 3, 4, 5, 6], ["nicht entsch."], ["Absent"], null, null, true),
+        };
+
+        var row = Assert.Single(ClassTeacherRosterRow.Build(students, [], [],
+            new DateOnly(2026, 8, 26), yearAbsences, schoolDaysElapsed: 20));
+
+        Assert.True(row.HasYearSummary);
+        Assert.Equal(2, row.YearAbsenceDayCount);
+        Assert.Equal(1, row.YearUnexcusedDayCount);
+        Assert.Equal(10, row.YearAbsenceRatePercent);
+        Assert.Contains("10 %", row.YearSummaryLabel);
+        Assert.Contains("2 von 20", row.YearSummaryTooltip);
+        Assert.Contains("1 unentschuldigt", row.YearSummaryTooltip);
+    }
+
+    [Fact]
+    public void RosterBuild_OhneJahresdatenZeigtKeineFehlquote()
+    {
+        var students = new[] { Student(1001, "Ada Müller") };
+
+        var row = Assert.Single(ClassTeacherRosterRow.Build(students, [], [], new DateOnly(2026, 8, 26)));
+
+        Assert.False(row.HasYearSummary);
+        Assert.Equal("", row.YearSummaryLabel);
+        Assert.Null(row.YearSummaryTooltip);
+    }
+
+    [Fact]
+    public void RosterBuild_OrdnetJahresfehlzeitenAuchOhneExternKeyUeberNamenZu()
+    {
+        var students = new[] { Student(externKey: null, displayName: "Ben Schmidt") };
+        var yearAbsences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 10), "Schmidt Ben", null, 2, 90,
+                ["Che"], [1, 2], ["entsch."], ["Krank"], null, null, false),
+        };
+
+        var row = Assert.Single(ClassTeacherRosterRow.Build(students, [], [], new DateOnly(2026, 8, 26),
+            yearAbsences, schoolDaysElapsed: 10));
+
+        Assert.Equal(1, row.YearAbsenceDayCount);
+    }
+
+    [Fact]
+    public void TrendDay_AnteileSummierenSichZurGesamtquote()
+    {
+        var day = new ClassTeacherTrendDay("Mo", 5, 2, 1, 2, 0.4d, 0.2d, 0.4d);
+
+        Assert.Equal(1.0d, day.TotalFraction, precision: 10);
+    }
+
+    [Fact]
+    public void RosterBuild_StelltGefahrUndWarnungMitSymbolVoran()
+    {
+        var students = new[] { Student(1001, "Ada Müller"), Student(1002, "Ben Schmidt") };
+        var todayAbsences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 26), "Müller Ada", 1001, 2, 90,
+                ["Che"], [1, 2], ["nicht entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 26), "Schmidt Ben", 1002, 2, 90,
+                ["Che"], [1, 2], ["entsch."], ["Absent"], null, null, false),
+        };
+
+        var roster = ClassTeacherRosterRow.Build(students, todayAbsences, [], new DateOnly(2026, 8, 26));
+
+        var ada = Assert.Single(roster, r => r.StudentName == "Ada Müller");
+        Assert.Equal(ClassTeacherStatusKind.Danger, ada.StatusKind);
+        Assert.StartsWith("✕ ", ada.StatusTextWithGlyph);
+
+        var ben = Assert.Single(roster, r => r.StudentName == "Ben Schmidt");
+        Assert.Equal(ClassTeacherStatusKind.Warning, ben.StatusKind);
+        Assert.StartsWith("△ ", ben.StatusTextWithGlyph);
+    }
+
+    [Fact]
+    public void RosterBuild_LaesstUnauffaelligenStatusOhneSymbol()
+    {
+        var students = new[] { Student(1001, "Zora Unauffällig") };
+
+        var row = Assert.Single(ClassTeacherRosterRow.Build(students, [], [], new DateOnly(2026, 8, 26)));
+
+        Assert.Equal(row.StatusText, row.StatusTextWithGlyph);
+    }
+
+    // ── Offene Entschuldigungen (Feature-Idee 2) ─────────────────────────────
+
+    [Fact]
+    public void OpenExcuseBuild_SortiertNachTagenOffenAbsteigendUndIgnoriertEntschuldigte()
+    {
+        var absences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 20), "Alt Ada", 1001, 2, 90,
+                ["Che"], [1, 2], ["nicht entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 25), "Neu Ben", 1002, 1, 45,
+                ["Deu"], [1], ["nicht entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 24), "Entschuldigt Cem", 1003, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+        };
+
+        var rows = ClassTeacherOpenExcuseRow.Build(absences, new DateOnly(2026, 8, 26));
+
+        // StudentName kommt aus StudentDisplayName (WebUntis liefert "Nachname Vorname",
+        // hier für die Anzeige auf "Vorname Nachname" gedreht - siehe ClassAbsenceDaySummaryRow).
+        Assert.Equal(["Ada Alt", "Ben Neu"], rows.Select(r => r.StudentName));
+        Assert.Equal(6, rows[0].DaysOpen);
+        Assert.True(rows[0].IsOverdue);
+        Assert.Equal(1, rows[1].DaysOpen);
+        Assert.False(rows[1].IsOverdue);
+    }
+
+    // ── Aggregierte Klassenbuch-Kategorien (Feature-Idee 5) ──────────────────
+
+    [Fact]
+    public void CategoryAggregateBuild_ZaehltProKategorieUndNenntTopSchueler()
+    {
+        var entries = new[]
+        {
+            new ClassTeacherClassRegisterRow(new DateOnly(2026, 8, 20), "Deu", "Schmidt Ben", "test",
+                "Hausaufgaben fehlen", "Negativ", "HA fehlt"),
+            new ClassTeacherClassRegisterRow(new DateOnly(2026, 8, 21), "Deu", "Schmidt Ben", "test",
+                "Hausaufgaben fehlen", "Negativ", "HA fehlt"),
+            new ClassTeacherClassRegisterRow(new DateOnly(2026, 8, 22), "Che", "Müller Ada", "test",
+                "Hausaufgaben fehlen", "Negativ", "HA fehlt"),
+            new ClassTeacherClassRegisterRow(new DateOnly(2026, 8, 23), "Che", "Müller Ada", "test",
+                "Lob", "Positiv", "Gute Mitarbeit"),
+        };
+
+        var rows = ClassTeacherCategoryAggregateRow.Build(entries);
+
+        Assert.Equal(["Hausaufgaben fehlen", "Lob"], rows.Select(r => r.CategoryName));
+        var homework = rows[0];
+        Assert.Equal(3, homework.Count);
+        Assert.Contains("2× Ben Schmidt", homework.TopStudentsLabel);
+        Assert.Contains("1× Ada Müller", homework.TopStudentsLabel);
+    }
+
+    // ── Erweiterte Mustererkennung: Wochentags-Häufung (Feature-Idee 4) ──────
+
+    [Fact]
+    public void DetectWeekdayPatterns_ErkenntDeutlicheHaeufungAnEinemWochentag()
+    {
+        // 24.08./31.08.2026 und 07.09.2026 sind allesamt Montage.
+        var absences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 24), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 31), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 9, 7), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+        };
+        var names = new Dictionary<string, string> { [UntisNameMatching.NameKey("Muster Mona")] = "Mona Muster" };
+
+        var notices = ClassTeacherOverviewViewModel.DetectWeekdayPatterns(
+            absences, names, new HashSet<string>());
+
+        var notice = Assert.Single(notices);
+        Assert.Equal("Mona Muster", notice.StudentName);
+        Assert.Contains("Montag", notice.Message);
+        Assert.Equal(ClassTeacherStatusKind.Info, notice.Kind);
+    }
+
+    [Fact]
+    public void DetectWeekdayPatterns_IgnoriertZuKleineStichprobe()
+    {
+        var absences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 24), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 31), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+        };
+
+        var notices = ClassTeacherOverviewViewModel.DetectWeekdayPatterns(
+            absences, new Dictionary<string, string>(), new HashSet<string>());
+
+        Assert.Empty(notices);
+    }
+
+    [Fact]
+    public void DetectWeekdayPatterns_LaesstBereitsGemeldeteSchuelerAus()
+    {
+        var absences = new[]
+        {
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 24), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 8, 31), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+            new ClassAbsenceDaySummaryRow(new DateOnly(2026, 9, 7), "Muster Mona", 1001, 1, 45,
+                ["Deu"], [1], ["entsch."], ["Absent"], null, null, false),
+        };
+        var names = new Dictionary<string, string> { [UntisNameMatching.NameKey("Muster Mona")] = "Mona Muster" };
+
+        var notices = ClassTeacherOverviewViewModel.DetectWeekdayPatterns(
+            absences, names, new HashSet<string> { "Mona Muster" });
+
+        Assert.Empty(notices);
+    }
+
+    // ── Noten-/Mitarbeit-Brücke: Namensabgleich (Feature-Idee 7) ─────────────
+
+    [Fact]
+    public void MatchStudent_FindetUeberVorUndNachnameTrotzKommaInFullName()
+    {
+        // Student.FullName liefert "Nachname, Vorname" (mit Komma) - ein Abgleich darüber würde
+        // UntisNameMatching.NameKey verfälschen ("Müller," bliebe eigenes Wort). MatchStudent
+        // muss deshalb FirstName/LastName getrennt verwenden, nicht FullName.
+        var students = new List<Student>
+        {
+            new() { FirstName = "Ada", LastName = "Müller" },
+            new() { FirstName = "Ben", LastName = "Schmidt" },
+        };
+
+        var match = ClassTeacherOverviewViewModel.MatchStudent("Müller Ada", students);
+
+        Assert.NotNull(match);
+        Assert.Equal("Ada", match!.FirstName);
+    }
+
+    [Fact]
+    public void MatchStudent_LiefertNullOhneTreffer()
+    {
+        var students = new List<Student> { new() { FirstName = "Ada", LastName = "Müller" } };
+
+        Assert.Null(ClassTeacherOverviewViewModel.MatchStudent("Unbekannt Xyz", students));
+    }
+
+    private static UntisStudentRosterCacheEntry Student(int? externKey, string displayName) =>
+        new() { ClassName = "6a", ExternKey = externKey, DisplayName = displayName };
+}

LehrerApp.Desktop.Tests/DashboardViewModelTests.cs

@@ -6,10 +6,27 @@ using Xunit;
 namespace LehrerApp.Desktop.Tests;
 
 /// Tests für 9.1 (heutige Stunden mit Uhrzeit und Raum im Dashboard) und 9.2 (Absprung von einer
-/// Stunde). Deckt nur die neue Zeit-/Raum-Auflösung ab, nicht die übrigen, unveränderten
+/// Stunde), außerdem die Kalender-Sitzungsanzeige (3.3.4) und die Mindeststichprobe der
-/// Dashboard-Bausteine (Kalender, offene Aufgaben, Fehlzeiten-Warnungen, ...).
+/// Fehlzeiten-Warnung (5.2.3). Deckt nicht die übrigen, unveränderten Dashboard-Bausteine
+/// (offene Aufgaben, ...).
 public sealed class DashboardViewModelTests
 {
+    [Fact]
+    public void LeereHinweisbereiche_WerdenAusgeblendetUndZusammenfassungBleibtKompakt()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var today = DateOnly.FromDateTime(DateTime.Today);
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today });
+
+        Assert.False(vm.ExcusesCard.EffectiveIsVisible);
+        Assert.False(vm.CorrectionsCard.EffectiveIsVisible);
+        Assert.False(vm.AlertsCard.EffectiveIsVisible);
+        Assert.Equal("0 offene Punkte", vm.AttentionSummary);
+        Assert.True(vm.TodayCard.EffectiveIsVisible);
+        Assert.True(vm.CalendarCard.EffectiveIsVisible);
+    }
+
     private static PeriodScheduleService NewPeriodSchedule()
     {
         var tempPath = System.IO.Path.Combine(
@@ -18,17 +35,44 @@ public sealed class DashboardViewModelTests
         return new PeriodScheduleService(tempPath);
     }
 
-    private static DashboardViewModel BuildVm(LearningGroup group, Lesson lesson,
+    private static DashboardSettingsService NewDashboardSettings()
-        FakeTimetableSlots? slots = null, PeriodScheduleService? periodSchedule = null)
     {
-        var lessons = new FakeLessons();
+        var tempPath = System.IO.Path.Combine(
+            System.IO.Path.GetTempPath(), $"lehrerapp-dashboardsettings-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new DashboardSettingsService(tempPath);
+    }
+
+    private static SchoolCalendarSettingsService NewCalendarSettings()
+    {
+        var tempPath = System.IO.Path.Combine(
+            System.IO.Path.GetTempPath(), $"lehrerapp-schoolcalendar-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new SchoolCalendarSettingsService(tempPath);
+    }
+
+    private static DashboardViewModel BuildVm(LearningGroup group, Lesson lesson,
+        FakeTimetableSlots? slots = null, PeriodScheduleService? periodSchedule = null,
+        FakeExams? exams = null, FakeResults? results = null, FakeGrades? grades = null,
+        FakeReportGrades? reportGrades = null, FakeMemberships? memberships = null,
+        FakeWorkTasks? tasks = null, FakeStudents? students = null, FakeDocumentation? documentation = null,
+        DashboardSettingsService? dashboardSettings = null, FakeSchoolHolidays? schoolHolidays = null,
+        FakeLessons? lessons = null, FakeSubstitutionEntries? substitutions = null,
+        FakeSessions? sessions = null, FakeEntries? entries = null,
+        FakeAnnualPlanEvents? annualPlanEvents = null)
+    {
+        lessons ??= new FakeLessons();
         lessons.Add(lesson);
         return new DashboardViewModel(
             new FakeGroups([group]), new FakeSubjects([]), lessons,
-            new FakeExams([]), new FakeWorkTasks(), new FakeSessions([]), new FakeEntries(),
+            exams ?? new FakeExams([]), results ?? new FakeResults(), grades ?? new FakeGrades(),
-            new FakeStudents([]), new FakeDocumentation(),
+            reportGrades ?? new FakeReportGrades(), memberships ?? new FakeMemberships([]),
+            tasks ?? new FakeWorkTasks(), sessions ?? new FakeSessions([]), entries ?? new FakeEntries(),
+            students ?? new FakeStudents([]), documentation ?? new FakeDocumentation(),
             slots ?? new FakeTimetableSlots(), periodSchedule ?? NewPeriodSchedule(),
-            new AttendanceBalanceService(), new SchoolYearService());
+            new AttendanceBalanceService(), new SchoolYearService(), dashboardSettings ?? NewDashboardSettings(),
+            schoolHolidays ?? new FakeSchoolHolidays(), new PublicHolidayService(), NewCalendarSettings(),
+            substitutions ?? new FakeSubstitutionEntries(), annualPlanEvents);
     }
 
     [Fact]
@@ -108,4 +152,409 @@ public sealed class DashboardViewModelTests
 
         Assert.Equal(group.Id, navigatedTo);
     }
+
+    [Fact]
+    public void UpcomingDates_BuendeltKlausurenFristenUndFoerderplanPruefungen()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var exams = new FakeExams([
+            new Exam { GroupId = group.Id, Title = "Chemie-Test", Date = today.AddDays(5) },
+        ]);
+        var tasks = new FakeWorkTasks();
+        tasks.Add(new WorkTask { Title = "Notenschluss", DueDate = today.AddDays(10) });
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var documentation = new FakeDocumentation();
+        documentation.Add(new Documentation
+        {
+            StudentId = student.Id, Type = DocumentationType.SupportPlan, Title = "Leseförderung",
+            SupportData = new SupportData { Status = SupportStatus.Active, ReviewDate = today.AddDays(3) },
+        });
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today }, exams: exams,
+            tasks: tasks, students: new FakeStudents([student]), documentation: documentation);
+
+        Assert.Contains(vm.UpcomingDates, i => i.Kind == UpcomingDateKind.Exam && i.Title == "Chemie-Test");
+        Assert.Contains(vm.UpcomingDates, i => i.Kind == UpcomingDateKind.Deadline && i.Title == "Notenschluss");
+        Assert.Contains(vm.UpcomingDates, i => i.Kind == UpcomingDateKind.SupportPlan && i.StudentId == student.Id);
+    }
+
+    [Fact]
+    public void OpenCorrections_ZeigtBewertungsfortschrittJeKlausur()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var exam = new Exam
+            { GroupId = group.Id, Title = "Klausur 1", Date = today.AddDays(-2), Status = ExamStatus.Conducted };
+        var anna = new Student { FirstName = "Anna", LastName = "A" };
+        var ben = new Student { FirstName = "Ben", LastName = "B" };
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = group.Id, StudentId = anna.Id },
+            new GroupMembership { GroupId = group.Id, StudentId = ben.Id },
+        ]);
+        var results = new FakeResults();
+        results.Add(new ExamResult { ExamId = exam.Id, StudentId = anna.Id, Grade = "2" });
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today },
+            exams: new FakeExams([exam]), results: results, memberships: memberships,
+            students: new FakeStudents([anna, ben]));
+
+        var correction = Assert.Single(vm.OpenCorrections);
+        Assert.Equal(1, correction.Completed);
+        Assert.Equal(2, correction.Total);
+        Assert.Equal(50, correction.Percent);
+    }
+
+    [Fact]
+    public void Alerts_ErkenntDeutlichenNotenabfall()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c", GradingSystem = GradingSystem.Grades1To6 };
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = group.Id, StudentId = student.Id },
+        ]);
+        var grades = new FakeGrades();
+        grades.Add(new Grade { GroupId = group.Id, StudentId = student.Id, Date = today.AddDays(-20), Value = "2" });
+        grades.Add(new Grade { GroupId = group.Id, StudentId = student.Id, Date = today.AddDays(-15), Value = "2" });
+        grades.Add(new Grade { GroupId = group.Id, StudentId = student.Id, Date = today.AddDays(-10), Value = "4" });
+        grades.Add(new Grade { GroupId = group.Id, StudentId = student.Id, Date = today.AddDays(-5), Value = "4" });
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today }, grades: grades,
+            memberships: memberships, students: new FakeStudents([student]));
+
+        Assert.Contains(vm.Alerts, a => a.StudentId == student.Id && a.KindLabel == "Notenabfall");
+    }
+
+    [Fact]
+    public void Kalenderauswahl_ZeigtTermineDesGeklicktenTages()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var examDate = today.AddDays(2);
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today },
+            exams: new FakeExams([new Exam { GroupId = group.Id, Title = "Test", Date = examDate }]));
+        var day = vm.CalendarDays.Single(d => d.Date == examDate);
+
+        vm.SelectCalendarDayCommand.Execute(day);
+
+        Assert.True(day.IsSelected);
+        Assert.Contains(vm.SelectedDayEvents, e => e.Kind == CalendarEventKind.Exam && e.Title == "Test");
+    }
+
+    [Fact]
+    public void Kalender_ZeigtMitarbeitssitzungenAlsEigenenTermintyp()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var sessionDate = today.AddDays(3);
+        var session = new ParticipationSession { GroupId = group.Id, Date = sessionDate, Comment = "Aufsatz" };
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today },
+            sessions: new FakeSessions([session]));
+
+        var day = vm.CalendarDays.Single(d => d.Date == sessionDate);
+        Assert.True(day.HasSession);
+
+        vm.SelectCalendarDayCommand.Execute(day);
+
+        Assert.Contains(vm.SelectedDayEvents,
+            e => e.Kind == CalendarEventKind.ParticipationSession && e.Subtitle == "Aufsatz");
+    }
+
+    [Fact]
+    public void Kalender_ZeigtMehrtaegigenJahresplanParallelZumUnterrichtAn()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var annualPlan = new FakeAnnualPlanEvents();
+        annualPlan.Add(new AnnualPlanEvent
+        {
+            ExternalId = "fahrt", Title = "Klassenfahrt 6a",
+            Description = "Aushang beachten", Location = "Jugendherberge",
+            CalendarGroup = "Lehrkräfte", StartDate = today, EndDate = today.AddDays(2),
+            StartTime = new TimeOnly(13, 30), EndTime = new TimeOnly(15, 0), IsAllDay = false,
+        });
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today, Topic = "Redox" },
+            annualPlanEvents: annualPlan);
+
+        var day = vm.CalendarDays.Single(d => d.Date == today);
+        Assert.True(day.HasLesson);
+        Assert.True(day.HasAnnualPlanEvent);
+
+        vm.SelectCalendarDayCommand.Execute(day);
+
+        Assert.Contains(vm.SelectedDayEvents, e => e.Kind == CalendarEventKind.Lesson);
+        var annual = Assert.Single(vm.SelectedDayEvents, e => e.Kind == CalendarEventKind.AnnualPlan);
+        Assert.Equal("Klassenfahrt 6a", annual.Title);
+        Assert.Equal("Aushang beachten", annual.Description);
+        Assert.Contains("13:30", annual.Subtitle);
+        Assert.Null(annual.GroupId);
+    }
+
+    [Fact]
+    public void Kalender_AusStundeErzeugteSitzungErzeugtKeinenDoppelteintrag()
+    {
+        // Regressionstest (Nutzer-Feedback): "Sitzung erzeugen" (3.3.1) verknüpft die Sitzung über
+        // LessonId mit der Stunde — dieselbe Unterrichtsstunde darf im Kalender nicht zusätzlich
+        // als eigener Sitzungstermin auftauchen.
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var lesson = new Lesson { GroupId = group.Id, Date = today, Topic = "Brechung" };
+        var session = new ParticipationSession { GroupId = group.Id, Date = today, LessonId = lesson.Id, Comment = "Brechung" };
+        var vm = BuildVm(group, lesson, sessions: new FakeSessions([session]));
+
+        var day = vm.CalendarDays.Single(d => d.Date == today);
+        Assert.True(day.HasLesson);
+        Assert.False(day.HasSession);
+
+        vm.SelectCalendarDayCommand.Execute(day);
+
+        Assert.Single(vm.SelectedDayEvents);
+        Assert.DoesNotContain(vm.SelectedDayEvents, e => e.Kind == CalendarEventKind.ParticipationSession);
+    }
+
+    [Fact]
+    public void Kalender_EigeneKlasseAmStundenplanTagOhneLessonZeigtDenRingTrotzdem()
+    {
+        // Regressionstest (Nutzer-Feedback): der "Meine Klasse"-Ring soll schon laut Stundenplan
+        // gelten, nicht erst, sobald für den Tag eine Lesson angelegt wurde.
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var tomorrow = today.AddDays(1);
+        var group = new LearningGroup { Name = "9c", IsOwnClass = true };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = tomorrow.DayOfWeek, PeriodNumber = 3 });
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today }, slots: slots);
+
+        var day = vm.CalendarDays.Single(d => d.Date == tomorrow);
+        Assert.True(day.IsOwnClassDay);
+    }
+
+    [Fact]
+    public void DashboardKacheln_SichtbarkeitUndReihenfolgeWerdenGespeichert()
+    {
+        var settings = NewDashboardSettings();
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today }, dashboardSettings: settings);
+        vm.TodayCard.IsVisible = false;
+        vm.MoveCardDownCommand.Execute(vm.TasksCard);
+
+        var saved = settings.Load();
+
+        Assert.False(saved.Single(c => c.Key == "today").IsVisible);
+        Assert.True(saved.FindIndex(c => c.Key == "tasks") > 1);
+    }
+
+    // ── Ungeplante Stunden (Nutzer-Feedback) ──────────────────────────────────
+
+    [Fact]
+    public void UnplannedLessons_StundeplanSlotOhneLessonMitThema_ErscheintInDerListe()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 1 });
+        // "lesson" liegt bewusst weit in der Vergangenheit, damit sie nicht in den heute/morgen-
+        // Vorgriff fällt und den Test verfälscht.
+        var pastLesson = new Lesson { GroupId = group.Id, Date = today.AddYears(-1), LessonNumber = 1, Topic = "Alt" };
+
+        var vm = BuildVm(group, pastLesson, slots: slots);
+
+        var item = Assert.Single(vm.UnplannedLessons);
+        Assert.Equal(group.Id, item.GroupId);
+        Assert.Equal(1, item.PeriodNumber);
+        Assert.Equal("Heute", item.DateDisplay);
+    }
+
+    [Fact]
+    public void UnplannedLessons_LessonMitThemaVorhanden_ErscheintNicht()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 1 });
+        var lesson = new Lesson { GroupId = group.Id, Date = today, LessonNumber = 1, Topic = "Redoxreaktionen" };
+
+        var vm = BuildVm(group, lesson, slots: slots);
+
+        Assert.Empty(vm.UnplannedLessons);
+    }
+
+    [Fact]
+    public void UnplannedLessons_GruppeOhnePlanungsbedarf_WirdIgnoriert()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "Klassenrat", RequiresLessonPlanning = false };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 1 });
+        var pastLesson = new Lesson { GroupId = group.Id, Date = today.AddYears(-1), LessonNumber = 1, Topic = "Alt" };
+
+        var vm = BuildVm(group, pastLesson, slots: slots);
+
+        Assert.Empty(vm.UnplannedLessons);
+    }
+
+    [Fact]
+    public void UnplannedLessons_Feiertag_WirdUebersprungen()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 1 });
+        var pastLesson = new Lesson { GroupId = group.Id, Date = today.AddYears(-1), LessonNumber = 1, Topic = "Alt" };
+        var schoolHolidays = new FakeSchoolHolidays();
+        schoolHolidays.Add(new SchoolHoliday { Name = "Ferien", StartDate = today, EndDate = today.AddDays(1) });
+
+        var vm = BuildVm(group, pastLesson, slots: slots, schoolHolidays: schoolHolidays);
+
+        Assert.Empty(vm.UnplannedLessons);
+    }
+
+    [Fact]
+    public void UnplannedLessons_DoppelstundeMitThemaAmErstenSlot_ZweiterSlotWirdNichtGemeldet()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 3 });
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 4 });
+        var lesson = new Lesson { GroupId = group.Id, Date = today, LessonNumber = 3, Topic = "Redox" };
+
+        var vm = BuildVm(group, lesson, slots: slots);
+
+        Assert.Empty(vm.UnplannedLessons);
+    }
+
+    [Fact]
+    public void UnplannedLessons_DoppelstundeOhneThema_BeideSlotsWerdenGemeldet()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 3 });
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 4 });
+        var lesson = new Lesson { GroupId = group.Id, Date = today, LessonNumber = 3, Topic = "" };
+
+        var vm = BuildVm(group, lesson, slots: slots);
+
+        Assert.Equal(2, vm.UnplannedLessons.Count);
+        Assert.Contains(vm.UnplannedLessons, i => i.PeriodNumber == 3);
+        Assert.Contains(vm.UnplannedLessons, i => i.PeriodNumber == 4);
+    }
+
+    [Fact]
+    public void UnplannedLessons_StundenausfallEingetragen_WirdNichtGemeldet()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 5 });
+        slots.Add(new TimetableSlot { GroupId = group.Id, Weekday = today.DayOfWeek, PeriodNumber = 6 });
+        var pastLesson = new Lesson { GroupId = group.Id, Date = today.AddYears(-1), LessonNumber = 5, Topic = "Alt" };
+        var substitutions = new FakeSubstitutionEntries();
+        substitutions.Add(new SubstitutionEntry { Date = today, Kind = SubstitutionKind.Cancelled, PeriodNumber = 5 });
+        substitutions.Add(new SubstitutionEntry { Date = today, Kind = SubstitutionKind.Cancelled, PeriodNumber = 6 });
+
+        var vm = BuildVm(group, pastLesson, slots: slots, substitutions: substitutions);
+
+        Assert.Empty(vm.UnplannedLessons);
+    }
+
+    [Fact]
+    public void AttendanceWarnings_KleineStichprobeMitHoherQuoteWirdNichtGemeldet()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var sessions = new FakeSessions([]);
+        var entries = new FakeEntries();
+
+        // Nur 3 erfasste Termine, alle unentschuldigt (100 %) — Stichprobe zu klein für eine Meldung.
+        for (var i = 0; i < 3; i++)
+        {
+            var session = new ParticipationSession { GroupId = group.Id, Date = today.AddDays(-i - 1) };
+            sessions.Save(session);
+            entries.Add(new ParticipationEntry { SessionId = session.Id, StudentId = student.Id, Attendance = AttendanceStatus.Unexcused });
+        }
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today },
+            students: new FakeStudents([student]), sessions: sessions, entries: entries);
+
+        Assert.Empty(vm.AttendanceWarnings);
+    }
+
+    [Fact]
+    public void AttendanceWarnings_MeldetHoheQuoteAbMindeststichprobe()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var group = new LearningGroup { Name = "9c" };
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var sessions = new FakeSessions([]);
+        var entries = new FakeEntries();
+
+        // 10 erfasste Termine, 3 davon unentschuldigt (30 % > 20 % Schwelle) — genug Stichprobe.
+        for (var i = 0; i < 10; i++)
+        {
+            var session = new ParticipationSession { GroupId = group.Id, Date = today.AddDays(-i - 1) };
+            sessions.Save(session);
+            var status = i < 3 ? AttendanceStatus.Unexcused : AttendanceStatus.Present;
+            entries.Add(new ParticipationEntry { SessionId = session.Id, StudentId = student.Id, Attendance = status });
+        }
+
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today },
+            students: new FakeStudents([student]), sessions: sessions, entries: entries);
+
+        var item = Assert.Single(vm.AttendanceWarnings);
+        Assert.Equal(student.FullName, item.StudentName);
+        Assert.Equal(30.0, item.AbsenceRatePercent);
+    }
+
+    [Fact]
+    public async Task AddTask_SpeichertErgebnisUndAktualisiertOffeneAufgaben()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var tasks = new FakeWorkTasks();
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today }, tasks: tasks);
+        vm.OnAddTask = startAsReminder => Task.FromResult<WorkTask?>(
+            new WorkTask { Title = "Ansage an die Klasse", Kind = startAsReminder ? TaskKind.Reminder : TaskKind.WorkItem });
+
+        await vm.AddReminderCommand.ExecuteAsync(null);
+
+        Assert.Single(tasks.GetAll());
+        Assert.Contains(vm.OpenTasks, t => t.Title == "Ansage an die Klasse" && t.IsReminder);
+    }
+
+    [Fact]
+    public async Task AddTask_OhneDelegatMachtNichts()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var tasks = new FakeWorkTasks();
+        var vm = BuildVm(group, new Lesson { GroupId = group.Id, Date = today }, tasks: tasks);
+
+        await vm.AddTaskCommand.ExecuteAsync(null);
+
+        Assert.Empty(tasks.GetAll());
+    }
+
+    [Fact]
+    public void WeatherWarningItem_BereitetAmtlicheWarnungFuerDashboardAuf()
+    {
+        var warning = new WeatherWarning
+        {
+            Event = "GEWITTER", Headline = "Amtliche Warnung vor Gewitter",
+            Description = "Es treten Gewitter auf.", Instruction = "Gebäude aufsuchen.",
+            Severity = "Severe", Onset = new DateTime(2026, 8, 23, 16, 0, 0, DateTimeKind.Utc),
+            Expires = new DateTime(2026, 8, 23, 18, 0, 0, DateTimeKind.Utc),
+        };
+
+        var item = new DashboardWeatherWarningItem(warning);
+
+        Assert.Equal("Amtliche Warnung vor Gewitter", item.Headline);
+        Assert.Equal("#D32F2F", item.SeverityColor);
+        Assert.Contains("Uhr", item.PeriodDisplay);
+    }
 }

LehrerApp.Desktop.Tests/DocumentationDialogViewModelTests.cs

@@ -137,6 +137,24 @@ public sealed class DocumentationDialogViewModelTests
         Assert.Equal("Gut verlaufen", vm.Result.ParentCallData.Impressions);
     }
 
+    [Fact]
+    public void Save_SitzplanEntwurf_WirdAbgeschlossenUndBehaeltStundenbezug()
+    {
+        var sessionId = Guid.NewGuid();
+        var existing = new Documentation
+        {
+            StudentId = Guid.NewGuid(), Type = DocumentationType.Incident,
+            Date = DateOnly.FromDateTime(DateTime.Today), Title = "Streit/Konflikt",
+            IsDraft = true, ParticipationSessionId = sessionId, Tags = ["Streit/Konflikt"],
+        };
+        var vm = new DocumentationDialogViewModel(existing.StudentId, existing, new FakeAttachmentStorage());
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.False(vm.Result!.IsDraft);
+        Assert.Equal(sessionId, vm.Result.ParticipationSessionId);
+    }
+
     [Fact]
     public void Save_Elternbrief_UebernimmtVersandUndRueckmeldung()
     {
@@ -187,6 +205,60 @@ public sealed class DocumentationDialogViewModelTests
         Assert.NotEqual("", vm.AttachmentError);
     }
 
+    [Fact]
+    public void OhneStudentOptions_CanPickStudentIstFalse()
+    {
+        var vm = new DocumentationDialogViewModel(Guid.NewGuid(), null, new FakeAttachmentStorage());
+
+        Assert.False(vm.CanPickStudent);
+    }
+
+    [Fact]
+    public void MitStudentOptions_OhneAuswahl_SaveSetztFehlerUndSpeichertNicht()
+    {
+        var anna = new StudentOption(Guid.NewGuid(), "Anna Beispiel");
+        var vm = new DocumentationDialogViewModel(Guid.Empty, null, new FakeAttachmentStorage(), [anna])
+        {
+            Title = "Vorfall", TypeName = "Vorkommnis",
+        };
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.Null(vm.Result);
+        Assert.NotEqual("", vm.StudentError);
+    }
+
+    [Fact]
+    public void MitStudentOptions_AuswahlGetroffen_SaveUebernimmtSchuelerUndGruppe()
+    {
+        var groupId = Guid.NewGuid();
+        var anna = new StudentOption(Guid.NewGuid(), "Anna Beispiel");
+        var ben = new StudentOption(Guid.NewGuid(), "Ben Muster");
+        var vm = new DocumentationDialogViewModel(Guid.Empty, null, new FakeAttachmentStorage(),
+            [anna, ben], groupId)
+        {
+            Title = "Vorfall", TypeName = "Vorkommnis", SelectedStudent = ben,
+        };
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.NotNull(vm.Result);
+        Assert.Equal(ben.Id, vm.Result!.StudentId);
+        Assert.Equal(groupId, vm.Result.GroupId);
+    }
+
+    [Fact]
+    public void MitStudentOptions_Bearbeiten_SchuelerIstVorausgewaehlt()
+    {
+        var anna = new StudentOption(Guid.NewGuid(), "Anna Beispiel");
+        var ben = new StudentOption(Guid.NewGuid(), "Ben Muster");
+        var existing = new Documentation { StudentId = ben.Id, Title = "Alt" };
+
+        var vm = new DocumentationDialogViewModel(ben.Id, existing, new FakeAttachmentStorage(), [anna, ben]);
+
+        Assert.Equal(ben, vm.SelectedStudent);
+    }
+
     [Fact]
     public void DiscardUnsavedAttachments_EntferntNurNieGespeicherteAnhaenge()
     {

LehrerApp.Desktop.Tests/ExamGradingDialogViewModelTests.cs

@@ -0,0 +1,65 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Core.Services;
+using LehrerApp.Desktop.ViewModels.Groups;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+/// Deckt 1.6.3 (Bulk-Aktion "Rest als abwesend markieren") ab — löst das Nutzer-Feedback, dass
+/// Schüler, die nie nachschreiben, sonst dauerhaft eine leere, unberührte Zeile hinterlassen.
+public sealed class ExamGradingDialogViewModelTests
+{
+    [Fact]
+    public void MarkRemainingAbsent_MarktNurUnberührteZeilenAbwesend()
+    {
+        var group = new LearningGroup { Name = "Q1 Chemie" };
+        var studentWithPoints = new Student { FirstName = "Anna", LastName = "Bauer" };
+        var untouchedStudent1 = new Student { FirstName = "Ben", LastName = "Cordes" };
+        var untouchedStudent2 = new Student { FirstName = "Cara", LastName = "Diehl" };
+        var students = new FakeStudents([studentWithPoints, untouchedStudent1, untouchedStudent2]);
+        var memberships = new FakeMemberships(
+        [
+            new GroupMembership { GroupId = group.Id, StudentId = studentWithPoints.Id },
+            new GroupMembership { GroupId = group.Id, StudentId = untouchedStudent1.Id },
+            new GroupMembership { GroupId = group.Id, StudentId = untouchedStudent2.Id },
+        ]);
+        var exam = new Exam
+        {
+            GroupId = group.Id,
+            Title = "Klausur 1",
+            Tasks = [new ExamTask { Nr = 1, MaxPoints = 10 }],
+            GradingKey = GradingService.DefaultKey1To6(),
+        };
+        var results = new FakeResults();
+
+        var vm = new ExamGradingDialogViewModel(results, students, memberships,
+            new GradingService(), exam, group.Id);
+
+        var rowWithPoints = vm.Rows.Single(r => r.StudentId == studentWithPoints.Id);
+        rowWithPoints.Cells[0].TrySetValue(7);
+
+        vm.MarkRemainingAbsentCommand.Execute(null);
+
+        Assert.False(rowWithPoints.Absent);
+        Assert.True(vm.Rows.Single(r => r.StudentId == untouchedStudent1.Id).Absent);
+        Assert.True(vm.Rows.Single(r => r.StudentId == untouchedStudent2.Id).Absent);
+    }
+
+    [Fact]
+    public void MarkRemainingAbsent_LässtBereitsAbwesendMarkierteUnverändert()
+    {
+        var group = new LearningGroup { Name = "Q1 Chemie" };
+        var student = new Student { FirstName = "Ben", LastName = "Cordes" };
+        var students = new FakeStudents([student]);
+        var memberships = new FakeMemberships(
+            [new GroupMembership { GroupId = group.Id, StudentId = student.Id }]);
+        var exam = new Exam { GroupId = group.Id, Title = "Klausur 1", Tasks = [new ExamTask { Nr = 1, MaxPoints = 10 }], GradingKey = GradingService.DefaultKey1To6() };
+        var vm = new ExamGradingDialogViewModel(new FakeResults(), students, memberships,
+            new GradingService(), exam, group.Id);
+
+        vm.Rows[0].Absent = true;
+        vm.MarkRemainingAbsentCommand.Execute(null);
+
+        Assert.True(vm.Rows[0].Absent);
+    }
+}

LehrerApp.Desktop.Tests/ExamsOverviewViewModelTests.cs

@@ -0,0 +1,89 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Core.Services;
+using LehrerApp.Desktop.ViewModels.Exams;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+/// Deckt 1.6 (Klausuren-Hauptseite) ab: Priorisierung end-to-end über die ViewModel-Schicht,
+/// Parallelkurs-Umschalter (1.6.4) und Genehmigung/Ankündigung (1.6.5).
+public sealed class ExamsOverviewViewModelTests
+{
+    private static ExamsOverviewViewModel BuildVm(List<LearningGroup> groups, List<Exam> exams,
+        FakeMemberships? memberships = null, FakeResults? results = null) =>
+        new(new FakeExams(exams), results ?? new FakeResults(), new FakeGroups(groups),
+            memberships ?? new FakeMemberships([]), new GradingService(), new SchoolYearService());
+
+    [Fact]
+    public void Load_UnbearbeiteteKorrekturStehtVorGeplanterKlausur()
+    {
+        var group = new LearningGroup { Name = "Q1 Chemie", SchoolYear = new SchoolYearService().CurrentSchoolYear() };
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var overdue = new Exam { GroupId = group.Id, Title = "Alte Klausur", Status = ExamStatus.Conducted, Date = today.AddDays(-10) };
+        var planned = new Exam { GroupId = group.Id, Title = "Neue Klausur", Status = ExamStatus.Planned, Date = today.AddDays(3) };
+
+        var vm = BuildVm([group], [overdue, planned]);
+        vm.LoadCommand.Execute(null);
+
+        Assert.Equal("Alte Klausur", vm.Rows[0].Title);
+        Assert.Equal("Neue Klausur", vm.Rows[1].Title);
+    }
+
+    [Fact]
+    public void Load_ParallelkurseWerdenAlsGeschwisterErkannt()
+    {
+        var groupA = new LearningGroup { Name = "Chemie GK Q1", SchoolYear = new SchoolYearService().CurrentSchoolYear() };
+        var groupB = new LearningGroup { Name = "Chemie GK Q1b", SchoolYear = groupA.SchoolYear };
+        var original = new Exam { GroupId = groupA.Id, Title = "Redox", Status = ExamStatus.Planned, Date = DateOnly.FromDateTime(DateTime.Today).AddDays(5) };
+        var duplicate = new Exam
+        {
+            GroupId = groupB.Id, Title = "Redox", Status = ExamStatus.Planned,
+            Date = DateOnly.FromDateTime(DateTime.Today).AddDays(6), SharedExamGroupId = original.Id,
+        };
+
+        var vm = BuildVm([groupA, groupB], [original, duplicate]);
+        vm.LoadCommand.Execute(null);
+
+        vm.SelectedRow = vm.Rows.Single(r => r.Exam.Id == original.Id);
+        Assert.True(vm.SelectedRow.HasSibling);
+        Assert.Single(vm.Siblings);
+        Assert.Equal(duplicate.Id, vm.Siblings[0].Exam.Id);
+    }
+
+    [Fact]
+    public void ToggleApproval_SetztUndLöschtDatum()
+    {
+        var group = new LearningGroup { Name = "Q1 Chemie", SchoolYear = new SchoolYearService().CurrentSchoolYear() };
+        var exam = new Exam { GroupId = group.Id, Title = "Klausur", Status = ExamStatus.Planned, Date = DateOnly.FromDateTime(DateTime.Today).AddDays(2) };
+        var vm = BuildVm([group], [exam]);
+        vm.LoadCommand.Execute(null);
+        vm.SelectedRow = vm.Rows[0];
+
+        vm.ToggleApprovalCommand.Execute(null);
+        Assert.NotNull(exam.ApprovalGrantedAt);
+        Assert.StartsWith("Genehmigt am", vm.ApprovalLabel);
+
+        vm.ToggleApprovalCommand.Execute(null);
+        Assert.Null(exam.ApprovalGrantedAt);
+        Assert.Equal("Genehmigung noch ausstehend", vm.ApprovalLabel);
+    }
+
+    [Fact]
+    public void Selection_ZeigtKorrekturfortschrittOderNotenspiegelJeNachStatus()
+    {
+        var group = new LearningGroup { Name = "Q1 Chemie", SchoolYear = new SchoolYearService().CurrentSchoolYear() };
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var conducted = new Exam { GroupId = group.Id, Title = "Läuft", Status = ExamStatus.Conducted, Date = today.AddDays(-2) };
+        var returned = new Exam { GroupId = group.Id, Title = "Fertig", Status = ExamStatus.Returned, Date = today.AddDays(-20) };
+        var vm = BuildVm([group], [conducted, returned]);
+        vm.LoadCommand.Execute(null);
+
+        vm.SelectedRow = vm.Rows.Single(r => r.Exam.Id == conducted.Id);
+        Assert.True(vm.ShowCorrectionProgress);
+        Assert.False(vm.ShowGradeSummary);
+
+        vm.SelectedRow = vm.Rows.Single(r => r.Exam.Id == returned.Id);
+        Assert.False(vm.ShowCorrectionProgress);
+        Assert.True(vm.ShowGradeSummary);
+    }
+}

LehrerApp.Desktop.Tests/ExportServiceTests.cs

@@ -0,0 +1,32 @@
+using LehrerApp.Desktop.Services;
+using System.Text;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class CsvBuilderTests
+{
+    [Fact]
+    public void AddRow_MaskiertTrennzeichen_AnfuehrungszeichenUndZeilenumbrueche()
+    {
+        var csv = new CsvBuilder()
+            .AddRow("Normal", "A;B", "Zitat \"wichtig\"", "Zeile 1\nZeile 2")
+            .ToString();
+
+        Assert.Contains("Normal;\"A;B\"", csv);
+        Assert.Contains("\"Zitat \"\"wichtig\"\"\"", csv);
+        Assert.Contains("\"Zeile 1\nZeile 2\"", csv);
+    }
+
+    [Fact]
+    public void CsvExportfile_BereinigtDateinameUndErgaenztEndungUndUtf8Bom()
+    {
+        var export = ExportFile.Csv("Export", "Auswertung/Chemie", "Änderung");
+        var bytes = export.Content.ToArray();
+        var preamble = new UTF8Encoding(encoderShouldEmitUTF8Identifier: true).GetPreamble();
+
+        Assert.Equal("Auswertung_Chemie.csv", export.SuggestedFileName);
+        Assert.Equal(preamble, bytes[..preamble.Length]);
+        Assert.Equal("Änderung", Encoding.UTF8.GetString(bytes[preamble.Length..]));
+    }
+}

LehrerApp.Desktop.Tests/Fakes.cs

@@ -1,6 +1,10 @@
 using LehrerApp.Core.Interfaces;
 using LehrerApp.Core.Models;
+using LehrerApp.Core.Services;
 using LehrerApp.Desktop.Services;
+using LehrerApp.Desktop.ViewModels.Settings;
+using LehrerApp.Sync;
+using LehrerApp.Sync.Crypto;
 
 namespace LehrerApp.Desktop.Tests;
 
@@ -26,6 +30,83 @@ public static class TestSupport
     public static AiPlanningService BuildAiPlanningService() => new(
         new HttpClient(), new FakeLessons(), new FakeGroups([]), new FakeSubjects([]),
         new FakeCompetencyDomains(), new FakeAlternativeLessonPaths([]));
+
+    /// Analog zu <see cref="BuildAiSettingsService"/>, eigenes Temp-Verzeichnis je Aufruf.
+    public static WebUntisSettingsService BuildWebUntisSettingsService()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-webuntissettings-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new WebUntisSettingsService(tempPath);
+    }
+
+    /// Analog zu den übrigen dateibasierten Feed-Einstellungen: eigenes Temp-Verzeichnis.
+    public static AnnualPlanSettingsService BuildAnnualPlanSettingsService()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-annualplansettings-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new AnnualPlanSettingsService(tempPath);
+    }
+
+    /// Analog zu <see cref="BuildAiSettingsService"/>, eigenes Temp-Verzeichnis je Aufruf.
+    public static SyncSettingsService BuildSyncSettingsService()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-syncsettings-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new SyncSettingsService(tempPath);
+    }
+
+    /// Kein echter HTTP-Aufruf, solange SyncSettingsService.IsLoggedIn false ist (siehe
+    /// BuildAiPlanningService).
+    public static SyncAuthService BuildSyncAuthService() => new(new HttpClient());
+
+    /// Eigenes Temp-Verzeichnis je Aufruf (echte, dateibasierte LiteDB wie bei EventQueue üblich).
+    public static EventQueue BuildEventQueue()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-eventqueue-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new EventQueue(Path.Combine(tempPath, "queue.db"));
+    }
+
+    /// Analog zu <see cref="BuildAiSettingsService"/>, eigenes Temp-Verzeichnis je Aufruf.
+    public static PeriodScheduleService BuildPeriodScheduleService()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-periodschedule-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new PeriodScheduleService(tempPath);
+    }
+
+    /// Analog zu <see cref="BuildAiSettingsService"/>, eigenes Temp-Verzeichnis je Aufruf.
+    public static AppLogger BuildAppLogger()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-applogger-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new AppLogger(tempPath);
+    }
+
+    /// Für Tests, die nicht speziell den "Schlüssel neu erzeugt"-Warnzustand prüfen.
+    public static SyncKeyStatus BuildSyncKeyStatus(bool keyWasRegenerated = false) => new(keyWasRegenerated);
+
+    /// Neue, leere Fakes je Aufruf - für Tests, die nicht speziell TrashViewModel-Verhalten prüfen.
+    public static TrashViewModel BuildTrashViewModel() => new(
+        new FakeTrash(), new FakeGrades(), new FakeWorkTasks(), new FakeTimeEntries(),
+        new FakeSeatingPlans(), new FakeGradingKeyTemplates());
+
+    /// Analog zu <see cref="BuildAiSettingsService"/>, eigenes Temp-Verzeichnis je Aufruf.
+    public static AppearanceSettingsService BuildAppearanceSettingsService()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-appearance-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new AppearanceSettingsService(tempPath);
+    }
+
+    /// Eigenes Temp-Verzeichnis je Aufruf, damit Tests sich nicht gegenseitig über dieselbe
+    /// sync.key stören.
+    public static SyncKeyRecoveryService BuildSyncKeyRecoveryService()
+    {
+        var tempPath = Path.Combine(Path.GetTempPath(), $"lehrerapp-synckeyrecovery-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(tempPath);
+        return new SyncKeyRecoveryService(SyncCrypto.GenerateKey(), Path.Combine(tempPath, "sync.key"));
+    }
 }
 
 public class FakeStudents(List<Student> all) : IStudentRepository
@@ -72,14 +153,29 @@ public class FakeSeatingPlans(List<SeatingPlan>? initial = null) : ISeatingPlanR
         _all.Add(plan);
     }
     public void Delete(Guid id) => _all.RemoveAll(p => p.Id == id);
+    // Trash-Mechanismus (14.3) lebt real in LiteDbContext.MoveToTrash/RestoreFromTrash - Fakes
+    // bilden ihn bewusst nicht nach, ViewModel-Tests brauchen das nicht.
+    public void Restore(Guid trashId) { }
+}
+
+public class FakeTrash(List<TrashedItem>? initial = null) : ITrashRepository
+{
+    private readonly List<TrashedItem> _all = initial ?? [];
+    public void Add(TrashedItem item) => _all.Add(item);
+    public List<TrashedItem> GetAll() => _all.OrderByDescending(t => t.DeletedAt).ToList();
+    public void PurgeOlderThan(DateTime cutoffUtc) => _all.RemoveAll(t => t.DeletedAt < cutoffUtc);
 }
 
 public class FakeSessions(List<ParticipationSession> all) : IParticipationSessionRepository
 {
     public List<ParticipationSession> GetByGroup(Guid groupId) => all.Where(s => s.GroupId == groupId).ToList();
     public ParticipationSession? GetById(Guid id) => all.FirstOrDefault(s => s.Id == id);
-    public void Save(ParticipationSession session) { }
+    public void Save(ParticipationSession session)
-    public void Delete(Guid id) { }
+    {
+        all.RemoveAll(s => s.Id == session.Id);
+        all.Add(session);
+    }
+    public void Delete(Guid id) => all.RemoveAll(s => s.Id == id);
 }
 
 public class FakeEntries : IParticipationRepository
@@ -135,6 +231,7 @@ public class FakeGrades : IGradeRepository
         _all.Add(grade);
     }
     public void Delete(Guid id) => _all.RemoveAll(g => g.Id == id);
+    public void Restore(Guid trashId) { } // siehe FakeSeatingPlans.Restore
 }
 
 public class FakeExams(List<Exam> all) : IExamRepository
@@ -172,6 +269,7 @@ public class FakeGradingKeyTemplates : IGradingKeyTemplateRepository
     public GradingKeyTemplate? GetById(Guid id) => _all.FirstOrDefault(t => t.Id == id);
     public void Save(GradingKeyTemplate template) { _all.RemoveAll(t => t.Id == template.Id); _all.Add(template); }
     public void Delete(Guid id) => _all.RemoveAll(t => t.Id == id);
+    public void Restore(Guid trashId) { } // siehe FakeSeatingPlans.Restore
 }
 
 public class FakeSchemes : IGradingSchemeRepository
@@ -340,18 +438,102 @@ public class FakeSubstitutionEntries : ISubstitutionEntryRepository
     public void Add(SubstitutionEntry e) => _all.Add(e);
     public List<SubstitutionEntry> GetAll() => _all.ToList();
     public List<SubstitutionEntry> GetByDate(DateOnly date) => _all.Where(e => e.Date == date).ToList();
+    public SubstitutionEntry? GetByExternalId(string externalId) => _all.FirstOrDefault(e => e.ExternalId == externalId);
     public void Save(SubstitutionEntry entry) { _all.RemoveAll(e => e.Id == entry.Id); _all.Add(entry); }
     public void Delete(Guid id) => _all.RemoveAll(e => e.Id == id);
 }
 
+public class FakeUntisSnapshots : IUntisSnapshotRepository
+{
+    private readonly List<UntisSnapshotEntry> _all = [];
+    public void Add(UntisSnapshotEntry e) => _all.Add(e);
+    public List<UntisSnapshotEntry> GetAll() => _all.ToList();
+    public void Save(UntisSnapshotEntry entry) { _all.RemoveAll(e => e.Id == entry.Id); _all.Add(entry); }
+    public void Delete(Guid id) => _all.RemoveAll(e => e.Id == id);
+}
+
+public class FakeUntisSlotMappings : IUntisSlotMappingRepository
+{
+    private readonly List<UntisSlotMapping> _all = [];
+    public void Add(UntisSlotMapping m) => _all.Add(m);
+    public List<UntisSlotMapping> GetAll() => _all.ToList();
+    public void Save(UntisSlotMapping mapping) { _all.RemoveAll(m => m.Id == mapping.Id); _all.Add(mapping); }
+    public void Delete(Guid id) => _all.RemoveAll(m => m.Id == id);
+}
+
+public class FakeAnnualPlanEvents : IAnnualPlanEventRepository
+{
+    private readonly List<AnnualPlanEvent> _all = [];
+    public void Add(AnnualPlanEvent e) => _all.Add(e);
+    public List<AnnualPlanEvent> GetAll() => _all.ToList();
+    public List<AnnualPlanEvent> GetByRange(DateOnly from, DateOnly to) =>
+        _all.Where(e => e.StartDate <= to && e.EndDate >= from).ToList();
+    public AnnualPlanEvent? GetByExternalId(string externalId) =>
+        _all.FirstOrDefault(e => e.ExternalId == externalId);
+    public void Save(AnnualPlanEvent entry) { _all.RemoveAll(e => e.Id == entry.Id); _all.Add(entry); }
+    public void Delete(Guid id) => _all.RemoveAll(e => e.Id == id);
+}
+
+public class FakeUntisAbsenceCache : IUntisAbsenceCacheRepository
+{
+    private readonly List<UntisAbsenceCacheEntry> _all = [];
+    public List<UntisAbsenceCacheEntry> GetByClassAndRange(string className, int startDate, int endDate) =>
+        _all.Where(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate).ToList();
+    public void ReplaceRange(string className, int startDate, int endDate, IEnumerable<UntisAbsenceCacheEntry> entries)
+    {
+        _all.RemoveAll(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate);
+        _all.AddRange(entries);
+    }
+    public void InsertRange(IEnumerable<UntisAbsenceCacheEntry> entries) => _all.AddRange(entries);
+}
+
+public class FakeUntisClassRegisterCache : IUntisClassRegisterCacheRepository
+{
+    private readonly List<UntisClassRegisterCacheEntry> _all = [];
+    public List<UntisClassRegisterCacheEntry> GetByClassAndRange(string className, int startDate, int endDate) =>
+        _all.Where(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate).ToList();
+    public void ReplaceRange(string className, int startDate, int endDate, IEnumerable<UntisClassRegisterCacheEntry> entries)
+    {
+        _all.RemoveAll(e => e.ClassName == className && e.Date >= startDate && e.Date <= endDate);
+        _all.AddRange(entries);
+    }
+    public void InsertRange(IEnumerable<UntisClassRegisterCacheEntry> entries) => _all.AddRange(entries);
+}
+
+public class FakeUntisStudentRosterCache : IUntisStudentRosterCacheRepository
+{
+    private readonly List<UntisStudentRosterCacheEntry> _all = [];
+    public List<UntisStudentRosterCacheEntry> GetByClass(string className) =>
+        _all.Where(e => e.ClassName == className).ToList();
+    public void ReplaceAll(string className, IEnumerable<UntisStudentRosterCacheEntry> entries)
+    {
+        _all.RemoveAll(e => e.ClassName == className);
+        _all.AddRange(entries);
+    }
+}
+
+public class FakeUntisCacheFetchStates : IUntisCacheFetchStateRepository
+{
+    private readonly List<UntisCacheFetchState> _all = [];
+    public UntisCacheFetchState? Get(string className, UntisCacheKind kind) =>
+        _all.FirstOrDefault(s => s.ClassName == className && s.Kind == kind);
+    public void Save(UntisCacheFetchState state)
+    {
+        _all.RemoveAll(s => s.ClassName == state.ClassName && s.Kind == state.Kind);
+        _all.Add(state);
+    }
+}
+
 public class FakeWorkTasks : IWorkTaskRepository
 {
     private readonly List<WorkTask> _all = [];
     public void Add(WorkTask t) => _all.Add(t);
     public List<WorkTask> GetByStatus(WorkTaskStatus status) => _all.Where(t => t.Status == status).ToList();
     public List<WorkTask> GetAll() => _all.ToList();
+    public List<WorkTask> GetByGroup(Guid groupId) => _all.Where(t => t.GroupId == groupId).ToList();
     public void Save(WorkTask task) { _all.RemoveAll(t => t.Id == task.Id); _all.Add(task); }
     public void Delete(Guid id) => _all.RemoveAll(t => t.Id == id);
+    public void Restore(Guid trashId) { } // siehe FakeSeatingPlans.Restore
 }
 
 public class FakeTimeEntries : ITimeEntryRepository
@@ -364,6 +546,7 @@ public class FakeTimeEntries : ITimeEntryRepository
     public List<TimeEntry> GetByTask(Guid taskId) => _all.Where(e => e.TaskId == taskId).ToList();
     public void Save(TimeEntry entry) { _all.RemoveAll(e => e.Id == entry.Id); _all.Add(entry); }
     public void Delete(Guid id) => _all.RemoveAll(e => e.Id == id);
+    public void Restore(Guid trashId) { } // siehe FakeSeatingPlans.Restore
 }
 
 public class FakeReportGrades : IReportGradeRepository

LehrerApp.Desktop.Tests/FractionWidthConverterTests.cs

@@ -0,0 +1,37 @@
+using System.Globalization;
+using LehrerApp.Desktop.Converters;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class FractionWidthConverterTests
+{
+    [Fact]
+    public void Convert_MultipliziertTrackbreiteMitAnteil()
+    {
+        var result = FractionWidthConverter.Instance.Convert(
+            [200d, 0.25d], typeof(double), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal(50d, result);
+    }
+
+    [Fact]
+    public void Convert_LiefertNullBreiteBeiUngueltigenWerten()
+    {
+        // Bounds.Width kann vor dem ersten Layout-Pass NaN sein (Element noch nicht gemessen) —
+        // darf nicht zu einer negativen oder undefinierten Breite führen.
+        Assert.Equal(0d, FractionWidthConverter.Instance.Convert(
+            [double.NaN, 0.5d], typeof(double), null, CultureInfo.InvariantCulture));
+        Assert.Equal(0d, FractionWidthConverter.Instance.Convert(
+            ["not a double", 0.5d], typeof(double), null, CultureInfo.InvariantCulture));
+    }
+
+    [Fact]
+    public void Convert_ClamptNegativeErgebnisseAufNull()
+    {
+        var result = FractionWidthConverter.Instance.Convert(
+            [100d, -0.2d], typeof(double), null, CultureInfo.InvariantCulture);
+
+        Assert.Equal(0d, result);
+    }
+}

LehrerApp.Desktop.Tests/GenerateLessonSeriesDialogViewModelTests.cs

@@ -85,7 +85,7 @@ public sealed class GenerateLessonSeriesDialogViewModelTests
     }
 
     [Fact]
-    public void Save_Doppelstunde_ErzeugtZweiStundenAmSelbenTagMitUnterschiedlicherStundennummer()
+    public void Save_Doppelstunde_ErzeugtNurEineStundeVerankertAnDerErstenPeriode()
     {
         var groupId = Guid.NewGuid();
         var unitId = Guid.NewGuid();
@@ -98,9 +98,50 @@ public sealed class GenerateLessonSeriesDialogViewModelTests
 
         vm.SaveCommand.Execute(null);
 
+        Assert.Equal(1, vm.Result!.Created);
+        var created = lessons.GetByGroupAndRange(groupId, from, from).Select(l => l.LessonNumber).ToList();
+        Assert.Equal([3], created);
+    }
+
+    [Fact]
+    public void Save_Dreifachstunde_ErzeugtNurEineStundeVerankertAnDerErstenPeriode()
+    {
+        var groupId = Guid.NewGuid();
+        var unitId = Guid.NewGuid();
+        var from = new DateOnly(2025, 9, 1);
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = from.DayOfWeek, PeriodNumber = 3 });
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = from.DayOfWeek, PeriodNumber = 4 });
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = from.DayOfWeek, PeriodNumber = 5 });
+        var lessons = new FakeLessons();
+        var vm = BuildVm(slots, lessons, new FakeSchoolHolidays(), unitId, groupId, from, from);
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.Equal(1, vm.Result!.Created);
+        var created = lessons.GetByGroupAndRange(groupId, from, from).Select(l => l.LessonNumber).ToList();
+        Assert.Equal([3], created);
+    }
+
+    [Fact]
+    public void Save_ZweiGetrennteEinzelstundenAmSelbenTagBleibenGetrennt()
+    {
+        // Perioden 1 und 3 sind NICHT aufeinanderfolgend (Periode 2 fehlt) — keine Doppelstunde,
+        // beide bekommen eine eigene Lesson.
+        var groupId = Guid.NewGuid();
+        var unitId = Guid.NewGuid();
+        var from = new DateOnly(2025, 9, 1);
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = from.DayOfWeek, PeriodNumber = 1 });
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = from.DayOfWeek, PeriodNumber = 3 });
+        var lessons = new FakeLessons();
+        var vm = BuildVm(slots, lessons, new FakeSchoolHolidays(), unitId, groupId, from, from);
+
+        vm.SaveCommand.Execute(null);
+
         Assert.Equal(2, vm.Result!.Created);
         var created = lessons.GetByGroupAndRange(groupId, from, from).Select(l => l.LessonNumber).OrderBy(n => n).ToList();
-        Assert.Equal([3, 4], created);
+        Assert.Equal([1, 3], created);
     }
 
     [Fact]

LehrerApp.Desktop.Tests/GlobalSearchViewModelTests.cs

@@ -0,0 +1,74 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.ViewModels;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class GlobalSearchViewModelTests
+{
+    [Fact]
+    public void LeereSuche_ZeigtSchnellaktionen()
+    {
+        var vm = BuildVm();
+
+        Assert.Collection(vm.Results,
+            item => Assert.Equal(GlobalSearchAction.NewTask, item.Action),
+            item => Assert.Equal(GlobalSearchAction.NewReminder, item.Action),
+            item => Assert.Equal(GlobalSearchAction.NewStudent, item.Action));
+        Assert.Same(vm.Results[0], vm.SelectedResult);
+    }
+
+    [Fact]
+    public void Suche_FindetSchuelerGruppeKlausurUndAufgabe()
+    {
+        var group = new LearningGroup { Name = "9 Chemie", SchoolYear = "2026/27", GradeLevel = 9 };
+        var student = new Student { FirstName = "Mia", LastName = "Chemie" };
+        var exam = new Exam { GroupId = group.Id, Title = "Chemie-Test", Date = new DateOnly(2026, 9, 1) };
+        var tasks = new FakeWorkTasks();
+        tasks.Add(new WorkTask { GroupId = group.Id, Title = "Chemie-Klausur korrigieren" });
+        var vm = BuildVm([student], [group], [exam], tasks);
+
+        vm.Query = "Chemie";
+
+        Assert.Contains(vm.Results, x => x.Kind == GlobalSearchResultKind.Student && x.EntityId == student.Id);
+        Assert.Contains(vm.Results, x => x.Kind == GlobalSearchResultKind.Group && x.EntityId == group.Id);
+        Assert.Contains(vm.Results, x => x.Kind == GlobalSearchResultKind.Exam && x.EntityId == exam.Id);
+        Assert.Contains(vm.Results, x => x.Kind == GlobalSearchResultKind.Task && x.Title.Contains("korrigieren"));
+    }
+
+    [Fact]
+    public async Task TrefferAusfuehren_NavigiertUndSchliesstPalette()
+    {
+        var student = new Student { FirstName = "Mia", LastName = "Muster" };
+        var vm = BuildVm([student]);
+        vm.Query = "Muster";
+        var result = Assert.Single(vm.Results, x => x.Kind == GlobalSearchResultKind.Student);
+        GlobalSearchResult? navigated = null;
+        var closed = false;
+        vm.OnNavigate = x => navigated = x;
+        vm.OnClose = () => closed = true;
+
+        await vm.ExecuteCommand.ExecuteAsync(result);
+
+        Assert.Same(result, navigated);
+        Assert.True(closed);
+    }
+
+    [Fact]
+    public async Task ErinnerungSchnellErfassen_StartetDialogAlsErinnerung()
+    {
+        var vm = BuildVm();
+        bool? reminder = null;
+        vm.OnQuickAddTask = value => { reminder = value; return Task.CompletedTask; };
+        var action = vm.Results.Single(x => x.Action == GlobalSearchAction.NewReminder);
+
+        await vm.ExecuteCommand.ExecuteAsync(action);
+
+        Assert.True(reminder);
+    }
+
+    private static GlobalSearchViewModel BuildVm(List<Student>? students = null,
+        List<LearningGroup>? groups = null, List<Exam>? exams = null, FakeWorkTasks? tasks = null) =>
+        new(new FakeStudents(students ?? []), new FakeGroups(groups ?? []),
+            new FakeExams(exams ?? []), tasks ?? new FakeWorkTasks());
+}

LehrerApp.Desktop.Tests/GroupDetailViewModelTests.cs

@@ -18,14 +18,18 @@ public sealed class GroupDetailViewModelTests
         var grades = new FakeGrades();
 
         var vm = new GroupDetailViewModel(groups, students, memberships, subjects, exams, grades, tasks,
+            new GroupOverviewViewModel(new FakeLessons(), exams, new FakeSessions([]), new FakeEntries(), students,
+                new FakeDocumentation(), tasks, new AttendanceBalanceService(), new SchoolYearService()),
             new ParticipationTabViewModel(new FakeSessions([]), new FakeEntries(), new FakeAspects(),
                 students, memberships, groups, new FakeCompetencyDomains()),
             new GradeOverviewTabViewModel(grades, exams, new FakeResults(), students, memberships, new GradingService()),
             new PlanningTabViewModel(new FakeUnits(), new FakeLessons(), groups, subjects,
-                new FakeCompetencyDomains(), TestSupport.BuildAiSettingsService()),
+                new FakeCompetencyDomains(), TestSupport.BuildAiSettingsService(), new FakeSessions([])),
             new CompetencyOverviewTabViewModel(new FakeUnits(), exams, new FakeResults(),
                 new FakeCompetencyDomains(), students, new CompetencyAnalysisService()),
-            new SeatingPlanTabViewModel(new FakeSeatingPlans(), students, memberships));
+            new SeatingPlanTabViewModel(new FakeSeatingPlans(), students, memberships,
+                new FakeSessions([]), new FakeEntries(), new FakeAspects()),
+            new GroupDocumentationTabViewModel(new FakeDocumentation(), students, groups));
 
         vm.LoadGroup(group.Id);
         vm.SelectedExam = vm.Exams.First(e => e.Id == exam.Id);

LehrerApp.Desktop.Tests/GroupDocumentationTabViewModelTests.cs

@@ -0,0 +1,147 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.ViewModels.Groups;
+using LehrerApp.Desktop.ViewModels.Students;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+/// Tests für den bisher als Platzhalter existierenden Gruppen-Tab "Dokumentation" (Nutzer-Feedback):
+/// listet Dokumentationseinträge aller Gruppen-Schüler, zeigt Einträge aus anderen Lerngruppen
+/// standardmäßig mit an (optisch abgesetzt statt versteckt), mit optionalem Filter darauf.
+public sealed class GroupDocumentationTabViewModelTests
+{
+    private static GroupDocumentationTabViewModel BuildVm(
+        List<Student> students, List<LearningGroup> groups, FakeDocumentation? docs = null)
+    {
+        var vm = new GroupDocumentationTabViewModel(docs ?? new FakeDocumentation(),
+            new FakeStudents(students), new FakeGroups(groups));
+        return vm;
+    }
+
+    [Fact]
+    public void Initialize_ZeigtEintraegeAllerGruppenschuelerSortiertNachDatumAbsteigend()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var ben = new Student { FirstName = "Ben", LastName = "Muster" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Alt", Date = new DateOnly(2025, 9, 1) });
+        docs.Add(new Documentation { StudentId = ben.Id, GroupId = group.Id, Title = "Neu", Date = new DateOnly(2025, 9, 10) });
+
+        var vm = BuildVm([anna, ben], [group], docs);
+        vm.Initialize(group.Id);
+
+        Assert.Equal(2, vm.Entries.Count);
+        Assert.Equal("Neu", vm.Entries[0].Model.Title);
+        Assert.Equal("Alt", vm.Entries[1].Model.Title);
+    }
+
+    [Fact]
+    public void Initialize_EintragAusAndererGruppe_WirdMitangezeigtAberAlsFremdMarkiert()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var otherGroup = new LearningGroup { Name = "Mathematik 9b" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = otherGroup.Id, Title = "Aus anderem Kurs", Date = new DateOnly(2025, 9, 1) });
+
+        var vm = BuildVm([anna], [group, otherGroup], docs);
+        vm.Initialize(group.Id);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.False(entry.IsOwnGroup);
+        Assert.Equal("Mathematik 9b", entry.OtherGroupLabel);
+        Assert.True(entry.ContentOpacity < 1.0);
+    }
+
+    [Fact]
+    public void Initialize_EintragOhneGruppe_GiltAlsEigen()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = null, Title = "Allgemein", Date = new DateOnly(2025, 9, 1) });
+
+        var vm = BuildVm([anna], [group], docs);
+        vm.Initialize(group.Id);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.True(entry.IsOwnGroup);
+    }
+
+    [Fact]
+    public void OnlyThisGroup_Aktiviert_BlendetEintraegeAusAnderenGruppenAus()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var otherGroup = new LearningGroup { Name = "Mathematik 9b" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Eigen", Date = new DateOnly(2025, 9, 1) });
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = otherGroup.Id, Title = "Fremd", Date = new DateOnly(2025, 9, 2) });
+
+        var vm = BuildVm([anna], [group, otherGroup], docs);
+        vm.Initialize(group.Id);
+        vm.OnlyThisGroup = true;
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.Equal("Eigen", entry.Model.Title);
+    }
+
+    [Fact]
+    public void SelectedStudentFilter_AufEinzelnenSchueler_FiltertListe()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var ben = new Student { FirstName = "Ben", LastName = "Muster" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Anna-Eintrag", Date = new DateOnly(2025, 9, 1) });
+        docs.Add(new Documentation { StudentId = ben.Id, GroupId = group.Id, Title = "Ben-Eintrag", Date = new DateOnly(2025, 9, 1) });
+
+        var vm = BuildVm([anna, ben], [group], docs);
+        vm.Initialize(group.Id);
+        vm.SelectedStudentFilter = vm.StudentFilterOptions.Single(s => s.Id == anna.Id);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.Equal("Anna-Eintrag", entry.Model.Title);
+    }
+
+    [Fact]
+    public async Task AddDocumentation_SpeichertErgebnisUndLaedtNeu()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        var vm = BuildVm([anna], [group], docs);
+        vm.Initialize(group.Id);
+
+        vm.OnEditDocumentation = (groupId, options, editing) =>
+        {
+            Assert.Equal(group.Id, groupId);
+            Assert.Contains(options, o => o.Id == anna.Id);
+            Assert.Null(editing);
+            return Task.FromResult<Documentation?>(new Documentation
+            { StudentId = anna.Id, GroupId = groupId, Title = "Neu", Date = new DateOnly(2025, 9, 1) });
+        };
+
+        await vm.AddDocumentationCommand.ExecuteAsync(null);
+
+        var entry = Assert.Single(vm.Entries);
+        Assert.Equal("Neu", entry.Model.Title);
+    }
+
+    [Fact]
+    public async Task DeleteDocumentation_NachBestaetigung_EntferntEintragAusListe()
+    {
+        var group = new LearningGroup { Name = "9c" };
+        var anna = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var docs = new FakeDocumentation();
+        docs.Add(new Documentation { StudentId = anna.Id, GroupId = group.Id, Title = "Weg", Date = new DateOnly(2025, 9, 1) });
+        var vm = BuildVm([anna], [group], docs);
+        vm.Initialize(group.Id);
+        vm.OnConfirmDeleteDocumentation = _ => Task.FromResult(true);
+
+        await vm.DeleteDocumentationCommand.ExecuteAsync(vm.Entries[0]);
+
+        Assert.Empty(vm.Entries);
+    }
+}

LehrerApp.Desktop.Tests/GroupListViewModelTests.cs

@@ -0,0 +1,73 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Core.Services;
+using LehrerApp.Desktop.ViewModels.Groups;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+/// <summary>Tests für die direkte, einlagige Lerngruppen-Navigation.</summary>
+public sealed class GroupListViewModelTests
+{
+    private static GroupListViewModel BuildVm(params LearningGroup[] groups) =>
+        new(new FakeGroups([.. groups]), new FakeSubjects([]), new SchoolYearService());
+
+    [Fact]
+    public void Gruppenkarte_OeffnetDirektDieUebersicht()
+    {
+        var group = CurrentGroup("9c");
+        var vm = BuildVm(group);
+        Guid? openedId = null;
+        int? openedTab = null;
+        vm.OnNavigateToDetail = (id, tab) => { openedId = id; openedTab = tab; };
+
+        Assert.Single(vm.Groups).OpenCommand.Execute(null);
+
+        Assert.Equal(group.Id, openedId);
+        Assert.Equal(0, openedTab);
+    }
+
+    [Fact]
+    public async Task Verwaltungsmenue_BearbeitetDieGewaehlteKarteOhneZwischenauswahl()
+    {
+        var first = CurrentGroup("9a");
+        var second = CurrentGroup("9b");
+        var vm = BuildVm(first, second);
+        Guid? editedId = null;
+        vm.OnEditGroup = id => { editedId = id; return Task.CompletedTask; };
+        var secondItem = vm.Groups.Single(x => x.Id == second.Id);
+
+        await secondItem.EditCommand.ExecuteAsync(null);
+
+        Assert.Equal(second.Id, editedId);
+    }
+
+    [Fact]
+    public void Suche_FiltertWeiterhinNachGruppenname()
+    {
+        var vm = BuildVm(CurrentGroup("9 Chemie"), CurrentGroup("10 Mathematik"));
+
+        vm.SearchText = "Chemie";
+
+        Assert.Equal("9 Chemie", Assert.Single(vm.Groups).Name);
+    }
+
+    [Fact]
+    public void Archivieren_WirktAufDieKarteUndEntferntSieAusDerAktivenListe()
+    {
+        var group = CurrentGroup("9c");
+        var vm = BuildVm(group);
+        var item = Assert.Single(vm.Groups);
+
+        item.ToggleArchiveCommand.Execute(null);
+
+        Assert.False(group.IsActive);
+        Assert.Empty(vm.Groups);
+    }
+
+    private static LearningGroup CurrentGroup(string name) => new()
+    {
+        Name = name,
+        SchoolYear = new SchoolYearService().CurrentSchoolYear(),
+        IsActive = true,
+    };
+}

LehrerApp.Desktop.Tests/GroupOverviewViewModelTests.cs

@@ -0,0 +1,368 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Core.Services;
+using LehrerApp.Desktop.ViewModels;
+using LehrerApp.Desktop.ViewModels.Groups;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+/// Tests für die Kurs-Übersicht: sie liest bewusst keine eigenen Daten, sondern zieht nur
+/// zusammen, was Planung/Klausuren/Mitarbeit/Dokumentation ohnehin schon verwalten.
+public sealed class GroupOverviewViewModelTests
+{
+    private static GroupOverviewViewModel NewVm(FakeLessons? lessons = null, FakeExams? exams = null,
+        FakeSessions? sessions = null, FakeEntries? entries = null, FakeStudents? students = null,
+        FakeDocumentation? documentation = null, FakeWorkTasks? tasks = null) =>
+        new(lessons ?? new FakeLessons(), exams ?? new FakeExams([]), sessions ?? new FakeSessions([]),
+            entries ?? new FakeEntries(), students ?? new FakeStudents([]), documentation ?? new FakeDocumentation(),
+            tasks ?? new FakeWorkTasks(), new AttendanceBalanceService(), new SchoolYearService());
+
+    private static (GroupOverviewViewModel Vm, FakeLessons Lessons, FakeExams Exams,
+        FakeSessions Sessions, FakeEntries Entries, FakeStudents Students, Guid GroupId) BuildScenario(
+        List<Lesson>? lessons = null, List<Exam>? exams = null)
+    {
+        var groupId = Guid.NewGuid();
+        var lessonsRepo = new FakeLessons();
+        foreach (var l in lessons ?? []) lessonsRepo.Add(l);
+        var examsRepo = new FakeExams(exams ?? []);
+        var sessions = new FakeSessions([]);
+        var entries = new FakeEntries();
+        var students = new FakeStudents([]);
+
+        var vm = NewVm(lessonsRepo, examsRepo, sessions, entries, students);
+        vm.Initialize(groupId, "Testkurs");
+
+        return (vm, lessonsRepo, examsRepo, sessions, entries, students, groupId);
+    }
+
+    [Fact]
+    public void NaechsteStunde_FindetDieNaechsteGeplanteStundeInnerhalbDesZeitfensters()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var lessons = new FakeLessons();
+        lessons.Add(new Lesson { GroupId = groupId, Date = today.AddDays(10), Topic = "Später", Status = LessonStatus.Planned });
+        lessons.Add(new Lesson { GroupId = groupId, Date = today.AddDays(3), Topic = "Brechung", Status = LessonStatus.Planned });
+        lessons.Add(new Lesson { GroupId = groupId, Date = today.AddDays(1), Topic = "Schon vorbei", Status = LessonStatus.Conducted });
+
+        var vm = NewVm(lessons: lessons);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.True(vm.HasNextLesson);
+        Assert.Contains("Brechung", vm.NextLessonLabel);
+    }
+
+    [Fact]
+    public void NaechsteStunde_OhneGeplanteStundeIstNichtVorhanden()
+    {
+        var (vm, _, _, _, _, _, _) = BuildScenario();
+        Assert.False(vm.HasNextLesson);
+    }
+
+    [Fact]
+    public void NaechsteKlausur_FindetFruehesteKuenftigeKlausur()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var exams = new FakeExams([
+            new Exam { GroupId = groupId, Date = today.AddDays(20), Title = "Später" },
+            new Exam { GroupId = groupId, Date = today.AddDays(5), Title = "Klassenarbeit 2" },
+            new Exam { GroupId = groupId, Date = today.AddDays(-3), Title = "Vergangen" },
+        ]);
+        var vm = NewVm(exams: exams);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.True(vm.HasNextExam);
+        Assert.Contains("Klassenarbeit 2", vm.NextExamLabel);
+    }
+
+    [Fact]
+    public void Mitarbeit_OhneSitzungenZeigtHinweisOhneWarnung()
+    {
+        var (vm, _, _, _, _, _, _) = BuildScenario();
+        Assert.Contains("Noch keine Mitarbeitssitzung", vm.ParticipationHintLabel);
+        Assert.False(vm.ParticipationHintIsStale);
+    }
+
+    [Fact]
+    public void Mitarbeit_LetzteSitzungVorMehrAls14TagenGiltAlsVeraltet()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var sessions = new FakeSessions([new ParticipationSession { GroupId = groupId, Date = today.AddDays(-20) }]);
+        var vm = NewVm(sessions: sessions);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.True(vm.ParticipationHintIsStale);
+    }
+
+    [Fact]
+    public void Mitarbeit_LetzteSitzungKuerzlichGiltNichtAlsVeraltet()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var sessions = new FakeSessions([new ParticipationSession { GroupId = groupId, Date = today.AddDays(-2) }]);
+        var vm = NewVm(sessions: sessions);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.False(vm.ParticipationHintIsStale);
+    }
+
+    [Fact]
+    public void OffeneHausaufgabenKontrolle_ZeigtUnkontrollierteHausaufgabeDerLetztenStunde()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var lessons = new FakeLessons();
+        lessons.Add(new Lesson
+        {
+            GroupId = groupId, Date = today.AddDays(-2), Homework = "S. 42 Nr. 3",
+            HomeworkChecked = false, HomeworkCheckDismissed = false,
+        });
+        var vm = NewVm(lessons: lessons);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.True(vm.HasOpenHomeworkCheck);
+        Assert.Contains("S. 42 Nr. 3", vm.OpenHomeworkCheckLabel);
+    }
+
+    [Fact]
+    public void OffeneHausaufgabenKontrolle_VerschwindetSobaldKontrolliert()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var lessons = new FakeLessons();
+        lessons.Add(new Lesson
+        {
+            GroupId = groupId, Date = today.AddDays(-2), Homework = "S. 42 Nr. 3",
+            HomeworkChecked = true,
+        });
+        var vm = NewVm(lessons: lessons);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.False(vm.HasOpenHomeworkCheck);
+    }
+
+    [Fact]
+    public void OffeneEntschuldigungen_ListetOffenePendenteEntschuldigungenUndErlaubtAufloesen()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var session = new ParticipationSession { GroupId = groupId, Date = today.AddDays(-2) };
+        var sessions = new FakeSessions([session]);
+        var entries = new FakeEntries();
+        var entry = new ParticipationEntry { SessionId = session.Id, StudentId = student.Id, Attendance = AttendanceStatus.ExcusePending };
+        entries.Add(entry);
+        var students = new FakeStudents([student]);
+
+        var vm = NewVm(sessions: sessions, entries: entries, students: students);
+        vm.Initialize(groupId, "Testkurs");
+
+        var item = Assert.Single(vm.OpenExcuses);
+        Assert.Equal(student.FullName, item.StudentName);
+
+        item.MarkExcusedCommand.Execute(null);
+
+        Assert.Empty(vm.OpenExcuses);
+        Assert.Equal(AttendanceStatus.Excused, entries.GetBySessionAndStudent(session.Id, student.Id)!.Attendance);
+    }
+
+    [Fact]
+    public void OffeneDokumentationen_ZaehltNurEntwuerfeDieserGruppeOderOhneGruppe()
+    {
+        var groupId = Guid.NewGuid();
+        var otherGroupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var students = new FakeStudents([student]);
+        var documentation = new FakeDocumentation();
+        documentation.Add(new Documentation { StudentId = student.Id, GroupId = groupId, IsDraft = true, Title = "Beobachtung" });
+        documentation.Add(new Documentation { StudentId = student.Id, GroupId = null, IsDraft = true, Title = "Ohne Gruppe" });
+        documentation.Add(new Documentation { StudentId = student.Id, GroupId = otherGroupId, IsDraft = true, Title = "Anderer Kurs" });
+        documentation.Add(new Documentation { StudentId = student.Id, GroupId = groupId, IsDraft = false, Title = "Fertig" });
+
+        var vm = NewVm(students: students, documentation: documentation);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.Equal(2, vm.DraftDocumentationCount);
+    }
+
+    [Fact]
+    public void AuffaelligeFehlzeiten_WirdErstAbMindeststichprobeGemeldet()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var students = new FakeStudents([student]);
+        var sessions = new FakeSessions([]);
+        var entries = new FakeEntries();
+
+        // Nur 3 erfasste Termine, davon 3 unentschuldigt (100 %) — trotz hoher Quote noch keine
+        // Meldung, weil die Stichprobe zu klein ist (Schuljahresbeginn-Problem).
+        for (var i = 0; i < 3; i++)
+        {
+            var session = new ParticipationSession { GroupId = groupId, Date = today.AddDays(-i - 1) };
+            sessions.Save(session);
+            entries.Add(new ParticipationEntry { SessionId = session.Id, StudentId = student.Id, Attendance = AttendanceStatus.Unexcused });
+        }
+
+        var vm = NewVm(sessions: sessions, entries: entries, students: students);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.False(vm.HasAttendanceNotices);
+    }
+
+    [Fact]
+    public void AuffaelligeFehlzeiten_MeldetHoheQuoteAbMindeststichprobe()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var students = new FakeStudents([student]);
+        var sessions = new FakeSessions([]);
+        var entries = new FakeEntries();
+
+        // 10 erfasste Termine, 3 davon unentschuldigt (30 % > 20 % Schwelle) — jetzt genug
+        // Stichprobe für eine Meldung.
+        for (var i = 0; i < 10; i++)
+        {
+            var session = new ParticipationSession { GroupId = groupId, Date = today.AddDays(-i - 1) };
+            sessions.Save(session);
+            var status = i < 3 ? AttendanceStatus.Unexcused : AttendanceStatus.Present;
+            entries.Add(new ParticipationEntry { SessionId = session.Id, StudentId = student.Id, Attendance = status });
+        }
+
+        var vm = NewVm(sessions: sessions, entries: entries, students: students);
+        vm.Initialize(groupId, "Testkurs");
+
+        var item = Assert.Single(vm.AttendanceNotices);
+        Assert.Equal(student.FullName, item.StudentName);
+        Assert.Equal(30.0, item.AbsenceRatePercent);
+    }
+
+    [Fact]
+    public void NavigationCommands_RufenDelegatMitPassendemTabIndexAuf()
+    {
+        var (vm, _, _, _, _, _, _) = BuildScenario();
+        var indices = new List<int>();
+        vm.OnNavigateToTab = idx => indices.Add(idx);
+
+        vm.NavigateToPlanningCommand.Execute(null);
+        vm.NavigateToExamsCommand.Execute(null);
+        vm.NavigateToParticipationCommand.Execute(null);
+        vm.NavigateToDocumentationCommand.Execute(null);
+
+        Assert.Equal([6, 4, 3, 8], indices);
+    }
+
+    [Fact]
+    public void FehlendeHausaufgaben_ListetNurSchuelerAusDerLetztenSitzung()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var studentMissing = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        var studentDone = new Student { FirstName = "Ben", LastName = "Muster" };
+        var students = new FakeStudents([studentMissing, studentDone]);
+        var sessions = new FakeSessions([]);
+        var entries = new FakeEntries();
+
+        var olderSession = new ParticipationSession { GroupId = groupId, Date = today.AddDays(-10) };
+        sessions.Save(olderSession);
+        entries.Add(new ParticipationEntry
+        {
+            SessionId = olderSession.Id, StudentId = studentDone.Id,
+            Homework = HomeworkStatus.MissingOpen, HomeworkMissing = true,
+        });
+
+        var lastSession = new ParticipationSession { GroupId = groupId, Date = today.AddDays(-1) };
+        sessions.Save(lastSession);
+        entries.Add(new ParticipationEntry
+        {
+            SessionId = lastSession.Id, StudentId = studentMissing.Id,
+            Homework = HomeworkStatus.MissingOverdue, HomeworkMissing = true,
+        });
+        entries.Add(new ParticipationEntry
+        {
+            SessionId = lastSession.Id, StudentId = studentDone.Id,
+            Homework = HomeworkStatus.Completed, HomeworkMissing = false,
+        });
+
+        var vm = NewVm(sessions: sessions, entries: entries, students: students);
+        vm.Initialize(groupId, "Testkurs");
+
+        var item = Assert.Single(vm.MissingHomeworkStudents);
+        Assert.Equal(studentMissing.FullName, item.StudentName);
+        Assert.Contains(lastSession.Date.ToString("dd.MM.yyyy"), vm.MissingHomeworkSessionLabel);
+    }
+
+    [Fact]
+    public void FehlendeHausaufgaben_OhneOffenePostenIstLeer()
+    {
+        var (vm, _, _, _, _, _, _) = BuildScenario();
+        Assert.False(vm.HasMissingHomework);
+    }
+
+    [Fact]
+    public void AnstehendeAufgaben_ZeigtNurOffeneAufgabenDieserGruppeSortiertNachFaelligkeit()
+    {
+        var today = DateOnly.FromDateTime(DateTime.Today);
+        var groupId = Guid.NewGuid();
+        var otherGroupId = Guid.NewGuid();
+        var tasks = new FakeWorkTasks();
+        tasks.Add(new WorkTask { GroupId = groupId, Title = "Später fällig", DueDate = today.AddDays(10), Status = WorkTaskStatus.Open });
+        tasks.Add(new WorkTask { GroupId = groupId, Title = "Bald fällig", DueDate = today.AddDays(1), Status = WorkTaskStatus.Open });
+        tasks.Add(new WorkTask { GroupId = groupId, Title = "Erledigt", DueDate = today, Status = WorkTaskStatus.Done });
+        tasks.Add(new WorkTask { GroupId = otherGroupId, Title = "Anderer Kurs", DueDate = today, Status = WorkTaskStatus.Open });
+
+        var vm = NewVm(tasks: tasks);
+        vm.Initialize(groupId, "Testkurs");
+
+        Assert.True(vm.HasGroupTasks);
+        Assert.Equal(["Bald fällig", "Später fällig"], vm.GroupTasks.Select(t => t.Title));
+    }
+
+    [Fact]
+    public void AnstehendeAufgaben_OhneAufgabenIstLeer()
+    {
+        var (vm, _, _, _, _, _, _) = BuildScenario();
+        Assert.False(vm.HasGroupTasks);
+    }
+
+    [Fact]
+    public void NavigateToWorkload_RuftDelegatAuf()
+    {
+        var (vm, _, _, _, _, _, _) = BuildScenario();
+        var called = false;
+        vm.OnNavigateToWorkload = () => called = true;
+
+        vm.NavigateToWorkloadCommand.Execute(null);
+
+        Assert.True(called);
+    }
+
+    [Fact]
+    public async Task AddGroupTask_SpeichertErgebnisMitGruppeUndAktualisiertListe()
+    {
+        var tasks = new FakeWorkTasks();
+        var (vm, _, _, _, _, _, groupId) = BuildScenario();
+        var vmWithTasks = NewVm(tasks: tasks);
+        vmWithTasks.Initialize(groupId, "Testkurs");
+        vmWithTasks.OnAddGroupTask = requestedGroupId =>
+            Task.FromResult<WorkTask?>(new WorkTask { Title = "Material einsammeln", GroupId = requestedGroupId });
+
+        await vmWithTasks.AddGroupTaskCommand.ExecuteAsync(null);
+
+        var saved = Assert.Single(tasks.GetByGroup(groupId));
+        Assert.Equal("Material einsammeln", saved.Title);
+        Assert.True(vmWithTasks.HasGroupTasks);
+        Assert.Equal("Material einsammeln", Assert.Single(vmWithTasks.GroupTasks).Title);
+    }
+
+    [Fact]
+    public async Task AddGroupTask_OhneDelegatMachtNichts()
+    {
+        var (vm, _, _, _, _, _, _) = BuildScenario();
+        await vm.AddGroupTaskCommand.ExecuteAsync(null);
+        Assert.False(vm.HasGroupTasks);
+    }
+}

LehrerApp.Desktop.Tests/HazardAssessmentWizardViewModelTests.cs

@@ -0,0 +1,397 @@
+using LehrerApp.Core.AiPlanning;
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.Services;
+using LehrerApp.Desktop.ViewModels.Groups;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class HazardAssessmentWizardViewModelTests
+{
+    private static HazardAssessmentWizardViewModel Build(HazardAssessment? editing = null) => new(editing, "10c");
+
+    private static string BuildTempPath()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"lehrerapp-gbuwizard-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(path);
+        return path;
+    }
+
+    [Fact]
+    public void Neu_StartetAufSchritt0OhneKiOhneEditingKennzeichnung()
+    {
+        var vm = Build();
+
+        Assert.Equal(0, vm.StepIndex);
+        Assert.True(vm.IsBasicsStep);
+        Assert.True(vm.IsFirstStep);
+        Assert.Equal("Gefährdungsbeurteilung erstellen", vm.DialogTitle);
+        Assert.False(vm.CanUseAi);
+    }
+
+    [Fact]
+    public void Next_OhneTitel_BleibtAufSchritt0UndSetztFehler()
+    {
+        var vm = Build();
+
+        vm.NextCommand.Execute(null);
+
+        Assert.Equal(0, vm.StepIndex);
+        Assert.NotEqual("", vm.TitleError);
+    }
+
+    [Fact]
+    public void Next_MitTitel_GehtEinenSchrittWeiterUndAktualisiertStepFlags()
+    {
+        var vm = Build();
+        vm.Title = "Natrium in Wasser";
+
+        vm.NextCommand.Execute(null);
+
+        Assert.Equal(1, vm.StepIndex);
+        Assert.True(vm.IsSubstancesStep);
+        Assert.False(vm.IsFirstStep);
+        Assert.Equal("", vm.TitleError);
+    }
+
+    [Fact]
+    public void Back_AufSchritt0_BleibtAufSchritt0()
+    {
+        var vm = Build();
+
+        vm.BackCommand.Execute(null);
+
+        Assert.Equal(0, vm.StepIndex);
+    }
+
+    [Fact]
+    public void DurchlaufAllerSchritte_LandetAufZusammenfassung()
+    {
+        var vm = Build();
+        vm.Title = "Natrium in Wasser";
+
+        for (var i = 0; i < vm.StepCount - 1; i++) vm.NextCommand.Execute(null);
+
+        Assert.True(vm.IsSummaryStep);
+        Assert.True(vm.IsLastStep);
+
+        vm.NextCommand.Execute(null);
+        Assert.True(vm.IsSummaryStep, "Weiter auf dem letzten Schritt darf nicht darüber hinausgehen.");
+    }
+
+    [Fact]
+    public void AddSubstance_UndRemove_AktualisierenDieListeUndHasSubstances()
+    {
+        var vm = Build();
+        Assert.False(vm.HasSubstances);
+
+        vm.AddSubstanceCommand.Execute(null);
+        Assert.True(vm.HasSubstances);
+        var item = vm.Substances[0];
+        item.Name = "Natrium";
+        item.Amount = "ca. 0,5 g";
+        item.PictogramOptions.First(o => o.Value == GhsPictogram.Flammable).IsSelected = true;
+
+        item.RemoveCommand.Execute(null);
+
+        Assert.False(vm.HasSubstances);
+        Assert.Empty(vm.Substances);
+    }
+
+    [Fact]
+    public void AddHazard_TrimmtUndLeertDasEingabefeld()
+    {
+        var vm = Build();
+
+        vm.NewHazard = "  Verätzungsgefahr  ";
+        vm.AddHazardCommand.Execute(null);
+
+        Assert.Equal(["Verätzungsgefahr"], vm.Hazards);
+        Assert.Equal("", vm.NewHazard);
+    }
+
+    [Fact]
+    public void AddHazard_MitLeeremTextTutNichts()
+    {
+        var vm = Build();
+
+        vm.NewHazard = "   ";
+        vm.AddHazardCommand.Execute(null);
+
+        Assert.Empty(vm.Hazards);
+    }
+
+    [Fact]
+    public void RemoveHazard_EntferntGenauDenGenanntenEintrag()
+    {
+        var vm = Build();
+        vm.NewHazard = "A"; vm.AddHazardCommand.Execute(null);
+        vm.NewHazard = "B"; vm.AddHazardCommand.Execute(null);
+
+        vm.RemoveHazardCommand.Execute("A");
+
+        Assert.Equal(["B"], vm.Hazards);
+    }
+
+    [Fact]
+    public void AddProtectiveMeasure_TrimmtUndLeertDasEingabefeld()
+    {
+        var vm = Build();
+
+        vm.NewProtectiveMeasure = "  Schutzbrille  ";
+        vm.AddProtectiveMeasureCommand.Execute(null);
+
+        Assert.Equal(["Schutzbrille"], vm.ProtectiveMeasures);
+        Assert.Equal("", vm.NewProtectiveMeasure);
+    }
+
+    [Fact]
+    public void Save_OhneTitel_SetztFehlerUndSpringtZurueckAufSchritt0()
+    {
+        var vm = Build();
+        vm.Title = "Etwas";
+        vm.NextCommand.Execute(null); // Schritt 1
+        vm.Title = "   ";
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.Null(vm.Result);
+        Assert.Equal(0, vm.StepIndex);
+        Assert.NotEqual("", vm.TitleError);
+    }
+
+    [Fact]
+    public void Save_BautVollstaendigesErgebnisAusAllenSchritten()
+    {
+        var vm = Build();
+        vm.Title = "Natrium in Wasser";
+        vm.GroupLabel = "10c";
+        vm.DateText = "12.09.2026";
+        vm.KindName = "Lehrerversuch";
+        vm.Procedure = "Kurzbeschreibung";
+        vm.AddSubstanceCommand.Execute(null);
+        vm.Substances[0].Name = "Natrium";
+        vm.Substances[0].Amount = "ca. 0,5 g";
+        vm.Substances[0].HStatements = "H260";
+        vm.Substances[0].PictogramOptions.First(o => o.Value == GhsPictogram.Flammable).IsSelected = true;
+        vm.NewHazard = "Verpuffung"; vm.AddHazardCommand.Execute(null);
+        vm.NewProtectiveMeasure = "Schutzbrille"; vm.AddProtectiveMeasureCommand.Execute(null);
+        vm.FirstAid = "Wasser spülen";
+        vm.Disposal = "Abreagieren lassen";
+        vm.Notes = "Hinweis";
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.NotNull(vm.Result);
+        var result = vm.Result!;
+        Assert.Equal("Natrium in Wasser", result.Title);
+        Assert.Equal("10c", result.GroupLabel);
+        Assert.Equal(new DateOnly(2026, 9, 12), result.Date);
+        Assert.Equal(ExperimentKind.Lehrerversuch, result.Kind);
+        var substance = Assert.Single(result.Substances);
+        Assert.Equal("Natrium", substance.Name);
+        Assert.Equal([GhsPictogram.Flammable], substance.GhsPictograms);
+        Assert.Equal(["Verpuffung"], result.Hazards);
+        Assert.Equal(["Schutzbrille"], result.ProtectiveMeasures);
+        Assert.Equal("Wasser spülen", result.FirstAid);
+        Assert.Equal("Abreagieren lassen", result.Disposal);
+        Assert.Equal("Hinweis", result.Notes);
+        Assert.False(result.IsAiAssisted);
+    }
+
+    [Fact]
+    public void Konstruktor_MitEditingLaedtAlleFelderVor()
+    {
+        var editing = new HazardAssessment
+        {
+            Title = "Bestehende GBU",
+            GroupLabel = "9a",
+            Date = new DateOnly(2026, 1, 15),
+            Kind = ExperimentKind.Schuelerversuch,
+            Procedure = "Ablauf",
+            Substances = [new HazardSubstance { Name = "Salzsäure", GhsPictograms = [GhsPictogram.Corrosive] }],
+            Hazards = ["Verätzung"],
+            ProtectiveMeasures = ["Handschuhe"],
+            FirstAid = "Erste Hilfe Text",
+            Disposal = "Entsorgungstext",
+            Notes = "Notiz",
+            IsAiAssisted = true,
+        };
+
+        var vm = Build(editing);
+
+        Assert.Equal("Gefährdungsbeurteilung bearbeiten", vm.DialogTitle);
+        Assert.Equal("Bestehende GBU", vm.Title);
+        Assert.Equal("9a", vm.GroupLabel);
+        Assert.Equal("15.01.2026", vm.DateText);
+        Assert.Equal("Schülerversuch", vm.KindName);
+        Assert.Equal("Ablauf", vm.Procedure);
+        var substance = Assert.Single(vm.Substances);
+        Assert.Equal("Salzsäure", substance.Name);
+        Assert.True(substance.PictogramOptions.Single(o => o.Value == GhsPictogram.Corrosive).IsSelected);
+        Assert.Equal(["Verätzung"], vm.Hazards);
+        Assert.Equal(["Handschuhe"], vm.ProtectiveMeasures);
+        Assert.Equal("Erste Hilfe Text", vm.FirstAid);
+        Assert.Equal("Entsorgungstext", vm.Disposal);
+        Assert.Equal("Notiz", vm.Notes);
+        Assert.True(vm.IsAiAssisted);
+    }
+
+    [Fact]
+    public void CanUseAi_OhneKiAbhaengigkeiten_IstFalseUndSubstanzenKoennenNichtRecherchieren()
+    {
+        var vm = Build();
+        vm.AddSubstanceCommand.Execute(null);
+
+        Assert.False(vm.CanUseAi);
+        Assert.False(vm.Substances[0].CanResearch);
+    }
+
+    [Fact]
+    public void CanUseAi_MitAngemeldeterKiUndUnitUndLesson_IstTrueUndSubstanzenKoennenRecherchieren()
+    {
+        var aiSettings = new AiSettingsService(BuildTempPath());
+        aiSettings.SetEnabled(true);
+        aiSettings.SetCredentialsAndToken("sebastian", "token-abc");
+        var ai = new AiPlanningService(new HttpClient(), new FakeLessons(), new FakeGroups([]),
+            new FakeSubjects([]), new FakeCompetencyDomains(), new FakeAlternativeLessonPaths([]));
+        var unit = new Unit { Title = "Redoxreaktionen" };
+        var lesson = new Lesson { Topic = "Natrium in Wasser" };
+
+        var vm = new HazardAssessmentWizardViewModel(null, "10c", ai, aiSettings, unit, lesson);
+        vm.AddSubstanceCommand.Execute(null);
+
+        Assert.True(vm.CanUseAi);
+        Assert.True(vm.Substances[0].CanResearch);
+    }
+}
+
+public sealed class HazardSubstanceEditItemTests
+{
+    [Fact]
+    public async Task Research_OhneOnResearch_TutNichtsUndBleibtOhneFehler()
+    {
+        var item = new HazardSubstanceEditItem { Name = "Natrium" };
+
+        await item.ResearchCommand.ExecuteAsync(null);
+
+        Assert.Equal("", item.ResearchError);
+        Assert.False(item.IsResearching);
+    }
+
+    [Fact]
+    public async Task Research_OhneNamen_SetztFehlerUndRuftDelegateNicht()
+    {
+        var called = false;
+        var item = new HazardSubstanceEditItem
+        {
+            Name = "  ",
+            OnResearch = _ => { called = true; return Task.FromResult(new AiSubstanceResearchResponse()); },
+        };
+
+        await item.ResearchCommand.ExecuteAsync(null);
+
+        Assert.False(called);
+        Assert.Equal("Stoffname eingeben.", item.ResearchError);
+    }
+
+    [Fact]
+    public async Task Research_UebernimmtAlleFelderAusDerAntwort()
+    {
+        var item = new HazardSubstanceEditItem { Name = "Natrium" };
+        item.OnResearch = name =>
+        {
+            Assert.Equal("Natrium", name);
+            return Task.FromResult(new AiSubstanceResearchResponse
+            {
+                Name = "Natrium",
+                Cas = "7440-23-5",
+                SignalWord = "Gefahr",
+                HStatements = "H260",
+                PStatements = "P223, P231+P232",
+                ActivityRestriction = "Für Schüler ab der 5. Klasse freigegeben",
+                GhsPictograms = ["GHS02", "GHS05"],
+            });
+        };
+
+        await item.ResearchCommand.ExecuteAsync(null);
+
+        Assert.Equal("Natrium", item.Name);
+        Assert.Equal("7440-23-5", item.Cas);
+        Assert.Equal("Gefahr", item.SignalWord);
+        Assert.Equal("H260", item.HStatements);
+        Assert.Equal("P223, P231+P232", item.PStatements);
+        Assert.Equal("Für Schüler ab der 5. Klasse freigegeben", item.ActivityRestriction);
+        Assert.True(item.PictogramOptions.Single(o => o.Value == GhsPictogram.Flammable).IsSelected);
+        Assert.True(item.PictogramOptions.Single(o => o.Value == GhsPictogram.Corrosive).IsSelected);
+        Assert.False(item.PictogramOptions.Single(o => o.Value == GhsPictogram.Toxic).IsSelected);
+        Assert.False(item.IsResearching);
+        Assert.Equal("", item.ResearchError);
+    }
+
+    [Fact]
+    public async Task Research_BeiFehler_SetztResearchErrorUndBehaeltAlteFelderUnveraendert()
+    {
+        var item = new HazardSubstanceEditItem { Name = "Unbekannt", HStatements = "H999" };
+        item.OnResearch = _ => throw new AiBackendException("Nicht genügend KI-Guthaben. Bitte Guthaben aufladen.");
+
+        await item.ResearchCommand.ExecuteAsync(null);
+
+        Assert.Equal("Nicht genügend KI-Guthaben. Bitte Guthaben aufladen.", item.ResearchError);
+        Assert.Equal("H999", item.HStatements);
+        Assert.False(item.IsResearching);
+    }
+}
+
+public sealed class ExperimentKindDisplayTests
+{
+    [Theory]
+    [InlineData(ExperimentKind.Lehrerversuch, "Lehrerversuch")]
+    [InlineData(ExperimentKind.Schuelerversuch, "Schülerversuch")]
+    [InlineData(ExperimentKind.Demonstrationsversuch, "Demonstrationsversuch")]
+    public void Label_UndFromLabel_SindZueinanderInvers(ExperimentKind kind, string label)
+    {
+        Assert.Equal(label, ExperimentKindDisplay.Label(kind));
+        Assert.Equal(kind, ExperimentKindDisplay.FromLabel(label));
+    }
+
+    [Fact]
+    public void FromLabel_UnbekanntFaelltAufLehrerversuchZurueck() =>
+        Assert.Equal(ExperimentKind.Lehrerversuch, ExperimentKindDisplay.FromLabel("Unbekannt"));
+
+    [Theory]
+    [InlineData("Schuelerversuch", ExperimentKind.Schuelerversuch)]
+    [InlineData("Demonstrationsversuch", ExperimentKind.Demonstrationsversuch)]
+    [InlineData("etwas anderes", ExperimentKind.Lehrerversuch)]
+    public void FromWireValue_LiestDenUnmarkiertenEnumNamenDerKiAntwort(string wire, ExperimentKind expected) =>
+        Assert.Equal(expected, ExperimentKindDisplay.FromWireValue(wire));
+}
+
+public sealed class GhsPictogramDisplayTests
+{
+    [Theory]
+    [InlineData(GhsPictogram.Explosive, "GHS01")]
+    [InlineData(GhsPictogram.Corrosive, "GHS05")]
+    [InlineData(GhsPictogram.Environmental, "GHS09")]
+    public void Code_LiefertDenErwartetenGhsCode(GhsPictogram value, string code) =>
+        Assert.Equal(code, GhsPictogramDisplay.Code(value));
+
+    [Fact]
+    public void FromCode_IstFallUnabhaengigUndTrimmt() =>
+        Assert.Equal(GhsPictogram.Corrosive, GhsPictogramDisplay.FromCode(" ghs05 "));
+
+    [Fact]
+    public void FromCode_UnbekannterCodeLiefertNull() =>
+        Assert.Null(GhsPictogramDisplay.FromCode("GHS99"));
+
+    [Fact]
+    public void BuildOptions_MarkiertNurDieUebergebenenAlsAusgewaehlt()
+    {
+        var options = GhsPictogramDisplay.BuildOptions([GhsPictogram.Toxic, GhsPictogram.Flammable]);
+
+        Assert.Equal(9, options.Count);
+        Assert.True(options.Single(o => o.Value == GhsPictogram.Toxic).IsSelected);
+        Assert.True(options.Single(o => o.Value == GhsPictogram.Flammable).IsSelected);
+        Assert.False(options.Single(o => o.Value == GhsPictogram.Corrosive).IsSelected);
+    }
+}

LehrerApp.Desktop.Tests/LessonDialogViewModelTests.cs

@@ -1,3 +1,4 @@
+using LehrerApp.Core.Interfaces;
 using LehrerApp.Core.Models;
 using LehrerApp.Core.Services;
 using LehrerApp.Desktop.ViewModels.Groups;
@@ -18,7 +19,7 @@ public sealed class LessonDialogViewModelTests
     private static LessonDialogViewModel BuildVm(Guid unitId, Guid groupId, Lesson? editing = null,
         FakeTimetableSlots? slots = null, PeriodScheduleService? periodSchedule = null) =>
         new(new FakeLessons(), new FakeShorthandCodes([]), new FakeAlternativeLessonPaths([]),
-            slots ?? new FakeTimetableSlots(), periodSchedule ?? NewPeriodSchedule(),
+            slots ?? new FakeTimetableSlots(), periodSchedule ?? NewPeriodSchedule(), new FakeAttachmentStorage(),
             unitId, groupId, "10c", "Chemie", [], [], editing);
 
     /// Nutzer-Feedback: bei einer Klasse, die in mehreren Fächern unterrichtet wird (mehrere
@@ -92,7 +93,7 @@ public sealed class LessonDialogViewModelTests
     {
         var codes = new FakeShorthandCodes([new ShorthandCode { Code = "Tb" }, new ShorthandCode { Code = "SH" }]);
         var vm = new LessonDialogViewModel(new FakeLessons(), codes, new FakeAlternativeLessonPaths([]),
-            new FakeTimetableSlots(), NewPeriodSchedule(),
+            new FakeTimetableSlots(), NewPeriodSchedule(), new FakeAttachmentStorage(),
             Guid.NewGuid(), Guid.NewGuid(), "10c", "Chemie", [], ["Plenum", "LDE", "Tb"], null); // "Tb" doppelt (Katalog + Historie), soll nur einmal erscheinen
 
         Assert.Equal(["LDE", "Plenum", "SH", "Tb"], vm.ShorthandSuggestions);
@@ -155,7 +156,7 @@ public sealed class LessonDialogViewModelTests
         var groupId = Guid.NewGuid();
         var lessons = new FakeLessons();
         var vm = new LessonDialogViewModel(lessons, new FakeShorthandCodes([]), new FakeAlternativeLessonPaths([]),
-            new FakeTimetableSlots(), NewPeriodSchedule(), unitId, groupId, "10c", "Chemie", [], [], null)
+            new FakeTimetableSlots(), NewPeriodSchedule(), new FakeAttachmentStorage(), unitId, groupId, "10c", "Chemie", [], [], null)
         {
             Topic = "Brechung", DateText = "01.09.2025", StartTimeText = "11:45",
         };
@@ -179,6 +180,111 @@ public sealed class LessonDialogViewModelTests
         Assert.Single(lessons.GetByUnit(unitId));
     }
 
+    [Fact]
+    public void AddAttachment_LaedtHochUndSaveUebernimmtIhnInDasErgebnis()
+    {
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid());
+        vm.Topic = "Thema"; vm.DateText = "01.09.2025";
+
+        vm.AddAttachment("gbu-natrium.pdf", new MemoryStream([1, 2, 3]));
+        vm.SaveCommand.Execute(null);
+
+        Assert.NotNull(vm.Result);
+        var attachment = Assert.Single(vm.Result!.Attachments);
+        Assert.Equal("gbu-natrium.pdf", attachment.FileName);
+        Assert.Equal(3, attachment.SizeBytes);
+    }
+
+    [Fact]
+    public void AddAttachment_ZuGrosseDateiWirdAbgelehnt()
+    {
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid());
+
+        vm.AddAttachment("riesig.pdf", new MemoryStream(new byte[IAttachmentStorage.MaxSizeBytes + 1]));
+
+        Assert.Empty(vm.Attachments);
+        Assert.NotEqual("", vm.AttachmentError);
+    }
+
+    [Fact]
+    public void RemoveAttachmentCommand_EntferntDenAnhangAusDerListe()
+    {
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid());
+        vm.AddAttachment("gbu-natrium.pdf", new MemoryStream([1, 2, 3]));
+        var item = vm.Attachments[0];
+
+        vm.RemoveAttachmentCommand.Execute(item);
+
+        Assert.Empty(vm.Attachments);
+    }
+
+    [Fact]
+    public void BearbeiteteStunde_LaedtVorhandeneAnhaengeVor()
+    {
+        var editing = new Lesson
+        {
+            Attachments = [new DocumentAttachment { StorageId = "abc", FileName = "gbu.pdf", SizeBytes = 42 }],
+        };
+
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid(), editing);
+
+        var attachment = Assert.Single(vm.Attachments);
+        Assert.Equal("gbu.pdf", attachment.FileName);
+    }
+
+    [Fact]
+    public void HasHomeworkText_OhneHausaufgabe_IstFalse()
+    {
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid());
+        Assert.False(vm.HasHomeworkText);
+
+        vm.Homework = "   ";
+        Assert.False(vm.HasHomeworkText);
+
+        vm.Homework = "S. 42";
+        Assert.True(vm.HasHomeworkText);
+    }
+
+    [Fact]
+    public void Save_UebernimmtHomeworkCheckedUndDismissed()
+    {
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid());
+        vm.Topic = "Brechung"; vm.DateText = "01.09.2025";
+        vm.Homework = "S. 42"; vm.HomeworkChecked = true;
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.True(vm.Result!.HomeworkChecked);
+        Assert.False(vm.Result.HomeworkCheckDismissed);
+    }
+
+    [Fact]
+    public void HomeworkChecked_UndHomeworkCheckDismissed_SchliessenSichGegenseitigAus()
+    {
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid());
+
+        vm.HomeworkChecked = true;
+        Assert.False(vm.HomeworkCheckDismissed);
+
+        vm.HomeworkCheckDismissed = true;
+        Assert.False(vm.HomeworkChecked);
+    }
+
+    [Fact]
+    public void Bearbeiten_UebernimmtGespeicherteHomeworkFlags()
+    {
+        var editing = new Lesson
+        {
+            Topic = "Alt", Date = new DateOnly(2025, 9, 1), Homework = "S. 42",
+            HomeworkCheckDismissed = true,
+        };
+
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid(), editing);
+
+        Assert.True(vm.HomeworkCheckDismissed);
+        Assert.False(vm.HomeworkChecked);
+    }
+
     [Fact]
     public void HasAlternativePath_AngehaktOeffnetZuweisungUndUebernimmtErgebnis()
     {
@@ -252,7 +358,7 @@ public sealed class LessonDialogViewModelTests
         };
 
         var vm = new LessonDialogViewModel(new FakeLessons(), new FakeShorthandCodes([]), alternativePaths,
-            new FakeTimetableSlots(), NewPeriodSchedule(), Guid.NewGuid(), Guid.NewGuid(), "10c", "Chemie", [], [], editing);
+            new FakeTimetableSlots(), NewPeriodSchedule(), new FakeAttachmentStorage(), Guid.NewGuid(), Guid.NewGuid(), "10c", "Chemie", [], [], editing);
 
         Assert.True(vm.Phases[0].HasAlternativePath);
         Assert.Equal("Kurzversion", vm.Phases[0].AlternativePathName);
@@ -388,4 +494,91 @@ public sealed class LessonDialogViewModelTests
 
         Assert.False(vm.HasTimeBudgetInfo);
     }
+
+    // ── Datumsvorschlag für eine neue Stunde (Nutzer-Feedback: Feld stand sonst immer auf
+    // "heute", was die Doppelstunden-Erkennung stillschweigend auf eine Einzelperiode
+    // zurückfallen ließ, wenn heute zufällig nicht der Unterrichtstag ist) ────────────────────
+
+    [Fact]
+    public void NeueStunde_OhneStundenplanEintraege_BleibtBeimHeutigenDatum()
+    {
+        var vm = BuildVm(Guid.NewGuid(), Guid.NewGuid()); // FakeTimetableSlots leer
+
+        Assert.Equal(DateOnly.FromDateTime(DateTime.Today).ToString("dd.MM.yyyy"), vm.DateText);
+    }
+
+    [Fact]
+    public void NeueStunde_LetzteStundeInDerZukunft_SchlaegtNaechstenPassendenWochentagDanachVor()
+    {
+        var unitId = Guid.NewGuid();
+        var groupId = Guid.NewGuid();
+        var futureLessonDate = new DateOnly(2099, 1, 6); // Dienstag
+        var lessons = new FakeLessons();
+        lessons.Add(new Lesson { UnitId = unitId, GroupId = groupId, Date = futureLessonDate, LessonNumber = 3 });
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = DayOfWeek.Tuesday, PeriodNumber = 3 });
+
+        var vm = new LessonDialogViewModel(lessons, new FakeShorthandCodes([]), new FakeAlternativeLessonPaths([]),
+            slots, NewPeriodSchedule(), new FakeAttachmentStorage(), unitId, groupId, "10c", "Chemie", [], [], null);
+
+        // Nächster Dienstag nach dem 06.01.2099 (einem Dienstag) ist der 13.01.2099.
+        Assert.Equal("13.01.2099", vm.DateText);
+        Assert.Equal(3, vm.LessonNumber);
+    }
+
+    [Fact]
+    public void NeueStunde_LetzteStundeInDerVergangenheit_SchlaegtNaechstenPassendenWochentagAbHeuteVor()
+    {
+        var unitId = Guid.NewGuid();
+        var groupId = Guid.NewGuid();
+        var lessons = new FakeLessons();
+        lessons.Add(new Lesson { UnitId = unitId, GroupId = groupId, Date = new DateOnly(2000, 1, 1), LessonNumber = 3 });
+        var weekday = DateTime.Today.DayOfWeek; // garantiert am selben oder folgenden Tag erreichbar
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = weekday, PeriodNumber = 3 });
+
+        var vm = new LessonDialogViewModel(lessons, new FakeShorthandCodes([]), new FakeAlternativeLessonPaths([]),
+            slots, NewPeriodSchedule(), new FakeAttachmentStorage(), unitId, groupId, "10c", "Chemie", [], [], null);
+
+        var suggested = DateOnly.ParseExact(vm.DateText, "dd.MM.yyyy");
+        Assert.True(suggested >= DateOnly.FromDateTime(DateTime.Today));
+        Assert.Equal(weekday, suggested.DayOfWeek);
+        Assert.Equal(3, vm.LessonNumber);
+    }
+
+    [Fact]
+    public void NeueStunde_Doppelstunde_SchlaegtDieErstePeriodeAlsStundennummerVor()
+    {
+        var unitId = Guid.NewGuid();
+        var groupId = Guid.NewGuid();
+        var futureLessonDate = new DateOnly(2099, 1, 6); // Dienstag
+        var lessons = new FakeLessons();
+        lessons.Add(new Lesson { UnitId = unitId, GroupId = groupId, Date = futureLessonDate, LessonNumber = 3 });
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = DayOfWeek.Tuesday, PeriodNumber = 4 });
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = DayOfWeek.Tuesday, PeriodNumber = 3 });
+
+        var vm = new LessonDialogViewModel(lessons, new FakeShorthandCodes([]), new FakeAlternativeLessonPaths([]),
+            slots, NewPeriodSchedule(), new FakeAttachmentStorage(), unitId, groupId, "10c", "Chemie", [], [], null);
+
+        Assert.Equal(3, vm.LessonNumber); // frühere Periode, nicht 4
+    }
+
+    [Fact]
+    public void NeueStunde_MitStundenvorschlag_UebernimmtBeginnAusStundenraster()
+    {
+        var unitId = Guid.NewGuid();
+        var groupId = Guid.NewGuid();
+        var weekday = DateTime.Today.DayOfWeek;
+        var slots = new FakeTimetableSlots();
+        slots.Add(new TimetableSlot { GroupId = groupId, Weekday = weekday, PeriodNumber = 2 });
+        var periodSchedule = NewPeriodSchedule();
+        periodSchedule.SetPeriods([new PeriodTimeEntry { PeriodNumber = 2, Start = new TimeOnly(8, 50), End = new TimeOnly(9, 35) }]);
+
+        var vm = new LessonDialogViewModel(new FakeLessons(), new FakeShorthandCodes([]), new FakeAlternativeLessonPaths([]),
+            slots, periodSchedule, new FakeAttachmentStorage(), unitId, groupId, "10c", "Chemie", [], [], null);
+
+        Assert.Equal(2, vm.LessonNumber);
+        Assert.Equal("08:50", vm.StartTimeText);
+    }
 }

LehrerApp.Desktop.Tests/LessonViewerViewModelTests.cs

@@ -97,4 +97,34 @@ public sealed class LessonViewerViewModelTests
         Assert.Equal("ab 12:00", vm.PhaseGroups[0].Phases[0].TimeDisplay);
         Assert.Equal("ab 12:00", vm.PhaseGroups[1].Phases[0].TimeDisplay);
     }
+
+    // ── Verzweigung aus dem Viewer (4.5.3) ────────────────────────────────────────
+
+    [Fact]
+    public void NavigateToParticipation_RuftDelegatMitTabIndexDreiAuf()
+    {
+        var groupId = Guid.NewGuid();
+        var lesson = new Lesson { GroupId = groupId, Topic = "Brechung", Date = new DateOnly(2025, 9, 1) };
+        var vm = new LessonViewerViewModel(lesson, new FakeAlternativeLessonPaths([]));
+        int? navigatedTab = null;
+        vm.OnNavigateToTab = idx => navigatedTab = idx;
+
+        vm.NavigateToParticipationCommand.Execute(null);
+
+        Assert.Equal(groupId, vm.GroupId);
+        Assert.Equal(3, navigatedTab);
+    }
+
+    [Fact]
+    public void NavigateToGrades_RuftDelegatMitTabIndexFuenfAuf()
+    {
+        var lesson = new Lesson { GroupId = Guid.NewGuid(), Topic = "Brechung", Date = new DateOnly(2025, 9, 1) };
+        var vm = new LessonViewerViewModel(lesson, new FakeAlternativeLessonPaths([]));
+        int? navigatedTab = null;
+        vm.OnNavigateToTab = idx => navigatedTab = idx;
+
+        vm.NavigateToGradesCommand.Execute(null);
+
+        Assert.Equal(5, navigatedTab);
+    }
 }

LehrerApp.Desktop.Tests/ParticipationSessionEditingTests.cs

@@ -0,0 +1,107 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.ViewModels.Groups;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class ParticipationSessionEditingTests
+{
+    [Fact]
+    public void Bearbeitungsdialog_AendertBestehendeSessionOhneVerknuepfungenZuErsetzen()
+    {
+        var session = new ParticipationSession
+        {
+            GroupId = Guid.NewGuid(),
+            Date = new DateOnly(2026, 8, 19),
+            Comment = "Sitzplan",
+            LessonId = Guid.NewGuid(),
+            CompetencyCodes = ["K1"],
+        };
+        var vm = new AddSessionDialogViewModel(session)
+        {
+            DateText = "20.08.2026",
+            Comment = "Elektrische Stromkreise",
+        };
+
+        vm.SaveCommand.Execute(null);
+
+        Assert.Same(session, vm.Result);
+        Assert.Equal(new DateOnly(2026, 8, 20), session.Date);
+        Assert.Equal("Elektrische Stromkreise", session.Comment);
+        Assert.NotNull(session.LessonId);
+        Assert.Equal(["K1"], session.CompetencyCodes);
+    }
+
+    [Fact]
+    public async Task BearbeitenBefehl_SpeichertSessionUndAktualisiertAnzeige()
+    {
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2026/27" };
+        var session = new ParticipationSession
+        {
+            GroupId = group.Id,
+            Date = new DateOnly(2026, 8, 19),
+            Comment = "Sitzplan",
+        };
+        var sessions = new FakeSessions([session]);
+        var vm = new ParticipationTabViewModel(
+            sessions, new FakeEntries(), new FakeAspects(), new FakeStudents([]),
+            new FakeMemberships([]), new FakeGroups([group]), new FakeCompetencyDomains());
+        vm.OnEditSession = existing =>
+        {
+            existing.Comment = "Nachbesprechung Elektrizität";
+            return Task.FromResult<ParticipationSession?>(existing);
+        };
+        vm.Initialize(group.Id, group.SchoolYear);
+
+        await vm.EditSessionCommand.ExecuteAsync(null);
+
+        Assert.Equal(session.Id, vm.SelectedSession?.Id);
+        Assert.Equal("Nachbesprechung Elektrizität", sessions.GetById(session.Id)?.Comment);
+        Assert.Contains("Nachbesprechung Elektrizität", vm.SelectedSessionDisplay);
+    }
+
+    [Fact]
+    public async Task LoeschenBefehl_FragtBestaetigungAbUndEntferntSessionMitsamtBewertungen()
+    {
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2026/27" };
+        var session = new ParticipationSession { GroupId = group.Id, Date = new DateOnly(2026, 8, 19) };
+        var sessions = new FakeSessions([session]);
+        var entries = new FakeEntries();
+        entries.Add(new ParticipationEntry { SessionId = session.Id, StudentId = Guid.NewGuid() });
+        var vm = new ParticipationTabViewModel(
+            sessions, entries, new FakeAspects(), new FakeStudents([]),
+            new FakeMemberships([]), new FakeGroups([group]), new FakeCompetencyDomains());
+        vm.Initialize(group.Id, group.SchoolYear);
+        vm.SelectedSession = vm.Sessions.Single(s => s.Id == session.Id);
+
+        int? passedEntryCount = null;
+        vm.OnConfirmDeleteSession = (_, count) =>
+        {
+            passedEntryCount = count;
+            return Task.FromResult(true);
+        };
+
+        await vm.DeleteSessionCommand.ExecuteAsync(null);
+
+        Assert.Equal(1, passedEntryCount);
+        Assert.Null(sessions.GetById(session.Id));
+    }
+
+    [Fact]
+    public async Task LoeschenBefehl_BrichtBeiAbgelehnterBestaetigungAb()
+    {
+        var group = new LearningGroup { Name = "8a", SchoolYear = "2026/27" };
+        var session = new ParticipationSession { GroupId = group.Id, Date = new DateOnly(2026, 8, 19) };
+        var sessions = new FakeSessions([session]);
+        var vm = new ParticipationTabViewModel(
+            sessions, new FakeEntries(), new FakeAspects(), new FakeStudents([]),
+            new FakeMemberships([]), new FakeGroups([group]), new FakeCompetencyDomains());
+        vm.Initialize(group.Id, group.SchoolYear);
+        vm.SelectedSession = vm.Sessions.Single(s => s.Id == session.Id);
+        vm.OnConfirmDeleteSession = (_, _) => Task.FromResult(false);
+
+        await vm.DeleteSessionCommand.ExecuteAsync(null);
+
+        Assert.NotNull(sessions.GetById(session.Id));
+    }
+}

LehrerApp.Desktop.Tests/PdfExportServiceTests.cs

@@ -0,0 +1,186 @@
+using LehrerApp.Desktop.Services;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class PdfExportServiceTests
+{
+    private static readonly PdfExportService Service = new();
+
+    private static void AssertIsPdf(byte[] bytes)
+    {
+        Assert.True(bytes.Length > 500, "PDF ist verdächtig klein.");
+        Assert.Equal("%PDF"u8.ToArray(), bytes[..4]);
+    }
+
+    [Fact]
+    public void BuildSeatingPlanPdf_VollstaendigerPlan_LiefertGueltigesPdf()
+    {
+        var data = new SeatingPlanPrintData("Standard", "Raum B204 · 2 × 3 Plätze",
+            Columns: 3, IsBoardAtBottom: false, ColumnGapWidths: [0, 90],
+            Seats:
+            [
+                new SeatPrintCell(0, 0, "Beispiel, Anna", false),
+                new SeatPrintCell(0, 1, null, false),
+                new SeatPrintCell(0, 2, "Muster, Ben", false),
+                new SeatPrintCell(1, 0, null, false),
+                new SeatPrintCell(1, 1, "Probe, Cem", false),
+                new SeatPrintCell(1, 2, null, false),
+            ]);
+
+        AssertIsPdf(Service.BuildSeatingPlanPdf(data));
+    }
+
+    /// Ausgeblendete Plätze (kein Tisch im Raum, siehe SeatingPlan.HiddenSeats) müssen als leere
+    /// Rasterposition mitgedruckt werden, ohne die Erzeugung zu stören.
+    [Fact]
+    public void BuildSeatingPlanPdf_MitAusgeblendetenPlaetzenUndTafelUnten_LiefertGueltigesPdf()
+    {
+        var data = new SeatingPlanPrintData("Physikraum", "",
+            Columns: 2, IsBoardAtBottom: true, ColumnGapWidths: [],
+            Seats:
+            [
+                new SeatPrintCell(0, 0, "Beispiel, Anna", false),
+                new SeatPrintCell(0, 1, null, true),
+                new SeatPrintCell(1, 0, null, true),
+                new SeatPrintCell(1, 1, null, false),
+            ]);
+
+        AssertIsPdf(Service.BuildSeatingPlanPdf(data));
+    }
+
+    [Fact]
+    public void BuildGradeListPdf_MatrixMitMehrerenSpalten_LiefertGueltigesPdf()
+    {
+        var data = new GradeListPrintData("8a Chemie", "2026/27", "1. Halbjahr",
+            ColumnHeaders: ["12.09. Klausur 1", "Mündlich 20.09.", "Hausaufgaben 01.10."],
+            Rows:
+            [
+                new GradeListPrintRow("Beispiel, Anna", ["2", "1", ""], "1.67"),
+                new GradeListPrintRow("Muster, Ben", ["abwesend", "3", "2"], "2.50"),
+                new GradeListPrintRow("Probe, Cem", ["", "", ""], "–"),
+            ]);
+
+        AssertIsPdf(Service.BuildGradeListPdf(data));
+    }
+
+    /// Ein Kurs kann übers Jahr viele Notenspalten ansammeln - die Tabelle muss auch dann noch
+    /// erzeugbar sein (QuestPDF staucht die Spalten, statt zu werfen).
+    [Fact]
+    public void BuildGradeListPdf_VieleSpalten_LiefertGueltigesPdf()
+    {
+        var headers = Enumerable.Range(1, 20).Select(i => $"Note {i}").ToList();
+        var rows = Enumerable.Range(1, 30).Select(i => new GradeListPrintRow(
+            $"Schüler {i}", [.. Enumerable.Repeat("2", 20)], "2.00")).ToList();
+
+        AssertIsPdf(Service.BuildGradeListPdf(new GradeListPrintData("LK Chemie", "2026/27",
+            "Gesamtes Schuljahr", headers, rows)));
+    }
+
+    [Fact]
+    public void BuildExamStatisticsPdf_MitVerteilungUndAufgabenanalyse_LiefertGueltigesPdf()
+    {
+        var data = new ExamStatisticsPrintData("Klausur 1 - Säuren und Basen", "12.09.2026", "E",
+            Stats:
+            [
+                new LabelValue("Bewertet", "24"),
+                new LabelValue("Abwesend", "2"),
+                new LabelValue("Durchschnitt", "2.71"),
+                new LabelValue("Median", "3"),
+                new LabelValue("Anteil nicht ausreichend (Note 5/6)", "8.3 %"),
+            ],
+            Distribution:
+            [
+                new GradeDistributionPrintRow("1", 3, 12.5),
+                new GradeDistributionPrintRow("2", 8, 33.3),
+                new GradeDistributionPrintRow("3", 7, 29.2),
+                new GradeDistributionPrintRow("4", 4, 16.7),
+                new GradeDistributionPrintRow("5", 2, 8.3),
+                new GradeDistributionPrintRow("6", 0, 0),
+            ],
+            TaskAnalysis:
+            [
+                new TaskAnalysisPrintRow("1. Titration", 78.4, false),
+                new TaskAnalysisPrintRow("2. pH-Berechnung", 42.1, true),
+            ]);
+
+        AssertIsPdf(Service.BuildExamStatisticsPdf(data));
+    }
+
+    /// Auch ohne Aufgabenstruktur (Klausur ohne Teilaufgaben) und ohne Niveau muss der
+    /// Notenspiegel druckbar sein - der Abschnitt Aufgabenanalyse entfällt dann einfach.
+    [Fact]
+    public void BuildExamStatisticsPdf_OhneAufgabenUndNiveau_LiefertGueltigesPdf()
+    {
+        var data = new ExamStatisticsPrintData("Test", "01.10.2026", "",
+            [new LabelValue("Bewertet", "0")], [], []);
+
+        AssertIsPdf(Service.BuildExamStatisticsPdf(data));
+    }
+
+    [Fact]
+    public void BuildCompetencyReportPdf_LiefertGueltigesPdf()
+    {
+        var data = new CompetencyReportPrintData("8a Chemie", "Beispiel, Anna",
+            "3 von 5 behandelt · 2 geprüft · 2 mit Ergebnisdaten", "Wiederholungsbedarf unter 60 %",
+            [
+                new CompetencyReportPrintRow("Stoff-Teilchen (ST)", "ST1",
+                    "Aggregatzustände auf Teilchenebene erklären", "2× behandelt", "1× geprüft",
+                    "72.5 % · 24 Schüler · 3 Aufgabenwerte", "85 % · 3 Aufgabenwerte", "Stand solide"),
+                new CompetencyReportPrintRow("Chemische Reaktion (CR)", "CR2",
+                    "Reaktionsgleichungen aufstellen", "1× behandelt · 1× geplant", "1× geprüft",
+                    "48.3 % · 24 Schüler · 2 Aufgabenwerte", "40 % · 2 Aufgabenwerte", "Wiederholungsbedarf"),
+                new CompetencyReportPrintRow("Energie (EN)", "–", "Energiediagramme deuten",
+                    "–", "1× geplant", "–", "–", "Noch keine Bewertung"),
+            ]);
+
+        AssertIsPdf(Service.BuildCompetencyReportPdf(data));
+    }
+
+    [Fact]
+    public void BuildStudentDocumentationPdf_MitVertraulichenUndEntwurfsEintraegen_LiefertGueltigesPdf()
+    {
+        var data = new StudentDocumentationPrintData("Beispiel, Anna",
+            [
+                new DocumentationPrintEntry("12.09.2026", "Beobachtung", "Unterricht gestört",
+                    "Wiederholt dazwischengerufen, nach Ermahnung gebessert.",
+                    ["Kritisch"], IsConfidential: false, IsDraft: false),
+                new DocumentationPrintEntry("20.09.2026", "Elterngespräch", "Telefonat Mutter",
+                    "Absprache: wöchentliche Rückmeldung im Hausaufgabenheft.",
+                    [], IsConfidential: true, IsDraft: false),
+                new DocumentationPrintEntry("01.10.2026", "Beobachtung", "Schnellnotiz",
+                    "", ["Nacharbeiten"], IsConfidential: false, IsDraft: true),
+            ]);
+
+        AssertIsPdf(Service.BuildStudentDocumentationPdf(data));
+    }
+
+    [Fact]
+    public void BuildHazardAssessmentPdf_MitStoffenUndKiHinweis_LiefertGueltigesPdf()
+    {
+        var data = new HazardAssessmentPrintData(
+            "Natrium in Wasser", "10c", "12.09.2026", "Lehrerversuch",
+            "Ein kleines Stück Natrium wird in eine Schale mit Wasser gegeben.",
+            [
+                new HazardSubstancePrintRow("Natrium", "7440-23-5", "ca. 0,5 g", "GHS02, GHS05", "Gefahr",
+                    "H260 - Reagiert mit Wasser unter Bildung entzündbarer Gase", "P223, P231+P232, P370+P378",
+                    "Abgabe an unter 18-Jährige verboten"),
+            ],
+            ["Wasserstoffbildung, Verpuffungsgefahr", "Verätzungsgefahr durch Natronlauge"],
+            ["Schutzbrille", "Sicherheitsabstand einhalten", "Abzug/Freiluftversuch"],
+            "Betroffene Stellen mit viel Wasser spülen, Arzt hinzuziehen.",
+            "Reste unter Aufsicht vollständig abreagieren lassen.",
+            "", IsAiAssisted: true);
+
+        AssertIsPdf(Service.BuildHazardAssessmentPdf(data));
+    }
+
+    [Fact]
+    public void BuildHazardAssessmentPdf_OhneStoffeUndOhneKiHinweis_LiefertGueltigesPdf()
+    {
+        var data = new HazardAssessmentPrintData(
+            "Papierchromatographie", "8b", "", "Schülerversuch", "", [], [], [], "", "", "", IsAiAssisted: false);
+
+        AssertIsPdf(Service.BuildHazardAssessmentPdf(data));
+    }
+}

LehrerApp.Desktop.Tests/PlanningExchangeServiceTests.cs

@@ -0,0 +1,130 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.Services;
+using System.Text.Json;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class PlanningExchangeServiceTests
+{
+    [Fact]
+    public void UnitRoundtrip_ErzeugtNeueObjekteUndErhaeltPlanungsdaten()
+    {
+        var units = new FakeUnits();
+        var lessons = new FakeLessons();
+        var shortPath = new AlternativeLessonPath { Name = "Kurzversion" };
+        var paths = new FakeAlternativeLessonPaths([shortPath]);
+        var service = new PlanningExchangeService(units, lessons, paths);
+        var sourceGroupId = Guid.NewGuid();
+        var targetGroupId = Guid.NewGuid();
+        var unit = new Unit
+        {
+            GroupId = sourceGroupId,
+            Title = "Optik",
+            StartDate = new DateOnly(2026, 9, 1),
+            EndDate = new DateOnly(2026, 10, 1),
+            Competencies = ["UF1", "E4"],
+            Status = UnitStatus.Active,
+            Notes = "Experimente",
+        };
+        units.Add(unit);
+        var sourceLesson = new Lesson
+        {
+            UnitId = unit.Id,
+            GroupId = sourceGroupId,
+            Date = new DateOnly(2026, 9, 8),
+            LessonNumber = 3,
+            Topic = "Reflexion",
+            StartTime = new TimeOnly(9, 50),
+            Status = LessonStatus.Planned,
+            Homework = "Aufgabe 2",
+            Phases =
+            [
+                new LessonPhaseStep
+                {
+                    Name = "Experiment", DurationMinutes = 25, Activity = "Winkel messen",
+                    Material = "Optikbox", Shorthand = "PA", AlternativePathId = shortPath.Id,
+                },
+            ],
+        };
+        lessons.Add(sourceLesson);
+
+        var json = service.ExportUnit(unit, new PlanningExchangeContext
+        {
+            Group = "8a", Subject = "Physik", GradeLevel = 8,
+        });
+        var result = service.ImportUnit(json, targetGroupId);
+
+        Assert.DoesNotContain(unit.Id.ToString(), json, StringComparison.OrdinalIgnoreCase);
+        Assert.DoesNotContain(sourceLesson.Id.ToString(), json, StringComparison.OrdinalIgnoreCase);
+        Assert.NotEqual(unit.Id, result.Unit.Id);
+        Assert.Equal(targetGroupId, result.Unit.GroupId);
+        Assert.Equal("Optik", result.Unit.Title);
+        Assert.Equal(UnitStatus.Active, result.Unit.Status);
+        Assert.Equal(["UF1", "E4"], result.Unit.Competencies);
+        var importedLesson = Assert.Single(result.Lessons);
+        Assert.NotEqual(sourceLesson.Id, importedLesson.Id);
+        Assert.Equal(result.Unit.Id, importedLesson.UnitId);
+        Assert.Equal(targetGroupId, importedLesson.GroupId);
+        Assert.Equal(new TimeOnly(9, 50), importedLesson.StartTime);
+        Assert.Equal(shortPath.Id, Assert.Single(importedLesson.Phases).AlternativePathId);
+    }
+
+    [Fact]
+    public void LessonImport_AkzeptiertMarkdownCodeblockUndLegtNeueStundeAn()
+    {
+        var units = new FakeUnits();
+        var lessons = new FakeLessons();
+        var service = new PlanningExchangeService(units, lessons, new FakeAlternativeLessonPaths([]));
+        var source = new Lesson
+        {
+            UnitId = Guid.NewGuid(), GroupId = Guid.NewGuid(), Date = new DateOnly(2026, 9, 8),
+            Topic = "Reflexion", Phases = [new LessonPhaseStep { Name = "Einstieg", DurationMinutes = 10 }],
+        };
+        lessons.Add(source);
+        var json = service.ExportLesson(source, new PlanningExchangeContext());
+        var targetUnitId = Guid.NewGuid();
+        var targetGroupId = Guid.NewGuid();
+
+        var imported = service.ImportLesson($"```json\n{json}\n```", targetUnitId, targetGroupId);
+
+        Assert.NotEqual(source.Id, imported.Id);
+        Assert.Equal(targetUnitId, imported.UnitId);
+        Assert.Equal(targetGroupId, imported.GroupId);
+        Assert.Equal("Reflexion", imported.Topic);
+        Assert.Equal(10, Assert.Single(imported.Phases).DurationMinutes);
+    }
+
+    [Fact]
+    public void ImportMitFalschemSchema_SchreibtKeineDaten()
+    {
+        var units = new FakeUnits();
+        var lessons = new FakeLessons();
+        var service = new PlanningExchangeService(units, lessons, new FakeAlternativeLessonPaths([]));
+        const string json = """
+            { "schema": "anderes.format", "version": 1, "unit": { "title": "Optik" } }
+            """;
+        var groupId = Guid.NewGuid();
+
+        var error = Assert.Throws<PlanningExchangeException>(() => service.ImportUnit(json, groupId));
+
+        Assert.Contains("Falsches Format", error.Message);
+        Assert.Empty(units.GetByGroup(groupId));
+        Assert.Empty(lessons.GetByGroupAndRange(groupId, DateOnly.MinValue, DateOnly.MaxValue));
+    }
+
+    [Fact]
+    public void ExportVerwendetDokumentiertesSchemaUndCamelCase()
+    {
+        var service = new PlanningExchangeService(new FakeUnits(), new FakeLessons(),
+            new FakeAlternativeLessonPaths([]));
+        var lesson = new Lesson { Date = new DateOnly(2026, 9, 8), Topic = "Optik" };
+
+        using var json = JsonDocument.Parse(service.ExportLesson(lesson, new PlanningExchangeContext()));
+
+        Assert.Equal(PlanningExchangeService.LessonSchema, json.RootElement.GetProperty("schema").GetString());
+        Assert.Equal(1, json.RootElement.GetProperty("version").GetInt32());
+        Assert.Equal("2026-09-08", json.RootElement.GetProperty("lesson").GetProperty("date").GetString());
+        Assert.Equal("planned", json.RootElement.GetProperty("lesson").GetProperty("status").GetString());
+    }
+}

LehrerApp.Desktop.Tests/PlanningTabViewModelTests.cs

@@ -20,7 +20,7 @@ public class PlanningTabViewModelTests
         var lessons = new FakeLessons();
 
         var vm = new PlanningTabViewModel(units, lessons, groups, subjects, competencyDomains,
-            TestSupport.BuildAiSettingsService());
+            TestSupport.BuildAiSettingsService(), new FakeSessions([]));
         vm.Initialize(groupId);
 
         return (vm, units, lessons, groupId);
@@ -227,4 +227,71 @@ public class PlanningTabViewModelTests
 
         Assert.Equal(["L->AB01", "Plenum"], vm.KnownShorthands);
     }
+
+    [Fact]
+    public void CreateParticipationSession_LegtSitzungMitDatumUndThemaDerStundeAn()
+    {
+        var groupId = Guid.NewGuid();
+        var group = new LearningGroup { Id = groupId, Name = "Testgruppe" };
+        var groups = new FakeGroups([group]);
+        var units = new FakeUnits();
+        var lessons = new FakeLessons();
+        var sessions = new FakeSessions([]);
+        var vm = new PlanningTabViewModel(units, lessons, groups, new FakeSubjects([]),
+            new FakeCompetencyDomains(), TestSupport.BuildAiSettingsService(), sessions);
+        vm.Initialize(groupId);
+
+        var unit = new Unit { GroupId = groupId, Title = "Optik" };
+        units.Add(unit);
+        var lesson = new Lesson
+        {
+            UnitId = unit.Id, GroupId = groupId, Date = new DateOnly(2025, 9, 1), Topic = "Brechung",
+        };
+        lessons.Add(lesson);
+        vm.RefreshPlanning(unit.Id, lesson.Id);
+        vm.SelectedLesson = vm.Lessons.Single(l => l.Id == lesson.Id);
+
+        string? notified = null;
+        vm.OnNotify = m => notified = m;
+
+        vm.CreateParticipationSessionCommand.Execute(null);
+
+        var created = Assert.Single(sessions.GetByGroup(groupId));
+        Assert.Equal(lesson.Id, created.LessonId);
+        Assert.Equal(lesson.Date, created.Date);
+        Assert.Equal("Brechung", created.Comment);
+        Assert.Equal("Sitzung aus der Stunde erstellt.", notified);
+    }
+
+    [Fact]
+    public void CreateParticipationSession_ZweiterAufrufLegtKeineDoppelteSitzungAn()
+    {
+        var groupId = Guid.NewGuid();
+        var group = new LearningGroup { Id = groupId, Name = "Testgruppe" };
+        var groups = new FakeGroups([group]);
+        var units = new FakeUnits();
+        var lessons = new FakeLessons();
+        var sessions = new FakeSessions([]);
+        var vm = new PlanningTabViewModel(units, lessons, groups, new FakeSubjects([]),
+            new FakeCompetencyDomains(), TestSupport.BuildAiSettingsService(), sessions);
+        vm.Initialize(groupId);
+
+        var unit = new Unit { GroupId = groupId, Title = "Optik" };
+        units.Add(unit);
+        var lesson = new Lesson
+        {
+            UnitId = unit.Id, GroupId = groupId, Date = new DateOnly(2025, 9, 1), Topic = "Brechung",
+        };
+        lessons.Add(lesson);
+        vm.RefreshPlanning(unit.Id, lesson.Id);
+        vm.SelectedLesson = vm.Lessons.Single(l => l.Id == lesson.Id);
+
+        vm.CreateParticipationSessionCommand.Execute(null);
+        string? notified = null;
+        vm.OnNotify = m => notified = m;
+        vm.CreateParticipationSessionCommand.Execute(null);
+
+        Assert.Single(sessions.GetByGroup(groupId));
+        Assert.Equal("Für diese Stunde existiert bereits eine Sitzung.", notified);
+    }
 }

LehrerApp.Desktop.Tests/QuickInputViewModelTests.cs

@@ -0,0 +1,104 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.ViewModels.Groups;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class QuickInputViewModelTests
+{
+    [Theory]
+    [InlineData(AttendanceStatus.Late, false, "Verspätet", "V")]
+    [InlineData(AttendanceStatus.SignificantlyLate, false, "Erheblich verspätet", "V!")]
+    [InlineData(AttendanceStatus.LeftDuringClass, true, "Während des Unterrichts abgängig", "A")]
+    [InlineData(AttendanceStatus.LearningIsland, true, "Lerninsel", "L")]
+    [InlineData(AttendanceStatus.Suspended, true, "Suspendiert", "S")]
+    public void ZusaetzlicheAnwesenheitsstatus_HabenAnzeigeUndPassendeAbwesenheitswertung(
+        AttendanceStatus status, bool isAbsent, string label, string symbol)
+    {
+        var row = new ParticipationStudentRow(Guid.NewGuid(), "Anna",
+            new ParticipationEntry { Attendance = status }, [], []);
+
+        Assert.Equal(isAbsent, row.IsAbsent);
+        Assert.Equal(label, row.AttendanceTooltip);
+        Assert.Equal(symbol, row.AttendanceLabel);
+        Assert.False(string.IsNullOrWhiteSpace(AttendanceDisplay.Color(status)));
+    }
+
+    [Fact]
+    public void ZurueckZuVorherigemSchueler_ZeigtBereitsGesetzteBewertung()
+    {
+        var aspects = new List<AspectColumnDef>
+        {
+            new(new ParticipationAspect { Key = "mitarbeit", Label = "Mitarbeit" }),
+        };
+        var rows = new List<ParticipationStudentRow>
+        {
+            new(Guid.NewGuid(), "Anna", new ParticipationEntry(), aspects, []),
+            new(Guid.NewGuid(), "Ben", new ParticipationEntry(), aspects, []),
+        };
+        var vm = new QuickInputViewModel(rows, aspects);
+
+        vm.SetRatingByNumber(4); // Scale5, 4. Stufe -> Rohwert 1 ("+"), Anna bewerten
+        vm.NextStudent();        // zu Ben
+        vm.PreviousStudent();    // zurück zu Anna
+
+        Assert.Equal(1, vm.AspectRows[0].Value);
+    }
+
+    [Fact]
+    public void AnwesenderSchueler_CurrentStudentIsAbsent_IstFalse()
+    {
+        var aspects = new List<AspectColumnDef>
+        {
+            new(new ParticipationAspect { Key = "a", Label = "A" }),
+        };
+        var rows = new List<ParticipationStudentRow>
+        {
+            new(Guid.NewGuid(), "Anna", new ParticipationEntry { Attendance = AttendanceStatus.Present }, aspects, []),
+        };
+        var vm = new QuickInputViewModel(rows, aspects);
+
+        Assert.False(vm.CurrentStudentIsAbsent);
+        Assert.Equal(1.0, vm.CurrentStudentContentOpacity);
+    }
+
+    [Fact]
+    public void AbwesenderSchueler_CurrentStudentIsAbsent_IstTrueUndGedimmt()
+    {
+        var aspects = new List<AspectColumnDef>
+        {
+            new(new ParticipationAspect { Key = "a", Label = "A" }),
+        };
+        var rows = new List<ParticipationStudentRow>
+        {
+            new(Guid.NewGuid(), "Anna", new ParticipationEntry(), aspects, []),
+            new(Guid.NewGuid(), "Ben", new ParticipationEntry { Attendance = AttendanceStatus.Unexcused }, aspects, []),
+        };
+        var vm = new QuickInputViewModel(rows, aspects);
+
+        vm.NextStudent(); // zu Ben, unentschuldigt abwesend
+
+        Assert.True(vm.CurrentStudentIsAbsent);
+        Assert.Equal("Krank, unentschuldigt", vm.CurrentStudentAttendanceLabel);
+        Assert.Equal(0.4, vm.CurrentStudentContentOpacity);
+    }
+
+    [Fact]
+    public void VorherigerAspekt_SpringtRueckwaertsMitUmlauf()
+    {
+        var aspects = new List<AspectColumnDef>
+        {
+            new(new ParticipationAspect { Key = "a", Label = "A" }),
+            new(new ParticipationAspect { Key = "b", Label = "B" }),
+        };
+        var rows = new List<ParticipationStudentRow>
+        {
+            new(Guid.NewGuid(), "Anna", new ParticipationEntry(), aspects, []),
+        };
+        var vm = new QuickInputViewModel(rows, aspects);
+
+        vm.PreviousAspect(); // von Index 0 rückwärts -> letzter Aspekt (Umlauf)
+
+        Assert.Equal(1, vm.AspectIndex);
+    }
+}

LehrerApp.Desktop.Tests/ReportGradeCalculationTests.cs

@@ -15,14 +15,15 @@ public class ReportGradeCalculationTests
 
     private static ReportGradeDialogViewModel BuildViewModel(
         FakeGrades grades, FakeSchemes schemes, FakeReportGrades reportGrades,
-        FakeExams exams, FakeResults results)
+        FakeExams exams, FakeResults results, FakeSessions? sessions = null, FakeEntries? entries = null)
     {
         var students = new FakeStudents([Anna, Ben]);
         var memberships = new FakeMemberships([]);
         var grading = new GradingService();
 
         return new ReportGradeDialogViewModel(grades, exams, results, students, memberships,
-            schemes, reportGrades, grading, GroupId, GroupType.Class, GradingSystem.Grades1To6,
+            schemes, reportGrades, sessions ?? new FakeSessions([]), entries ?? new FakeEntries(),
+            new AttendanceBalanceService(), grading, GroupId, GroupType.Class, GradingSystem.Grades1To6,
             "Testgruppe", "2025/26");
     }
 
@@ -68,6 +69,28 @@ public class ReportGradeCalculationTests
         Assert.Equal("–", ben.CalculatedDisplay);
     }
 
+    [Fact]
+    public void AbsenceRatePercent_BerechnetFehlquoteAusSitzungenDieserGruppeImZeitraum()
+    {
+        var (_, _, exams, results) = BuildExams();
+        var session1 = new ParticipationSession { GroupId = GroupId, Date = new DateOnly(2025, 9, 5) };
+        var session2 = new ParticipationSession { GroupId = GroupId, Date = new DateOnly(2025, 9, 12) };
+        var sessions = new FakeSessions([session1, session2]);
+        var entries = new FakeEntries();
+        entries.Add(new ParticipationEntry { SessionId = session1.Id, StudentId = Anna.Id, Attendance = AttendanceStatus.Present });
+        entries.Add(new ParticipationEntry { SessionId = session2.Id, StudentId = Anna.Id, Attendance = AttendanceStatus.Unexcused });
+
+        var vm = BuildViewModel(new FakeGrades(), new FakeSchemes(), new FakeReportGrades(), exams, results,
+            sessions, entries);
+
+        var anna = vm.Rows.Single(r => r.StudentId == Anna.Id);
+        Assert.Equal(50.0, anna.AbsenceRatePercent);
+        Assert.True(anna.HasHighAbsenceRate);
+        var ben = vm.Rows.Single(r => r.StudentId == Ben.Id);
+        Assert.Equal(0.0, ben.AbsenceRatePercent);
+        Assert.False(ben.HasHighAbsenceRate);
+    }
+
     [Fact]
     public void ResolveScheme_BevorzugtGruppenspezifischesSchemaVorVoreinstellung()
     {

LehrerApp.Desktop.Tests/SchoolDayWeatherSummaryTests.cs

@@ -0,0 +1,139 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Desktop.ViewModels.Planning;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class SchoolDayWeatherSummaryTests
+{
+    private static readonly DateOnly Day = new(2026, 8, 24);
+
+    [Fact]
+    public void From_GewitterHatVorrangVorSturmUndRegen()
+    {
+        var snapshot = Snapshot(
+            Hour(8, code: 61, rain: 2, gust: 70),
+            Hour(10, code: 95, rain: 1, gust: 80));
+
+        var summary = SchoolDayWeatherSummary.From(snapshot, Day);
+
+        Assert.Equal(SchoolDayWeatherKind.Thunderstorm, summary!.Kind);
+        Assert.Equal("⛈️", summary.Symbol);
+    }
+
+    [Fact]
+    public void From_BoeAbBeaufortAcht_WirdAlsSturmAngezeigt()
+    {
+        var summary = SchoolDayWeatherSummary.From(Snapshot(Hour(10, gust: 62)), Day);
+
+        Assert.Equal(SchoolDayWeatherKind.Storm, summary!.Kind);
+        Assert.Equal("💨", summary.Symbol);
+    }
+
+    [Fact]
+    public void From_NaechtlicheBoeBeeinflusstSchultagNicht()
+    {
+        var snapshot = Snapshot(Hour(10, cloud: 10), Hour(23, gust: 90));
+
+        var summary = SchoolDayWeatherSummary.From(snapshot, Day);
+
+        Assert.Equal(20, summary!.MaxWindGustKmh);
+        Assert.Equal(SchoolDayWeatherKind.Sunny, summary!.Kind);
+    }
+
+    [Fact]
+    public void From_VerwendetFuerKennzahlenNurAchtBisSechzehnUhr()
+    {
+        var snapshot = Snapshot(
+            Hour(7, temperature: 5, rain: 3, probability: 95, sunshineMinutes: 0),
+            Hour(8, temperature: 12, probability: 20, sunshineMinutes: 30),
+            Hour(16, temperature: 17, probability: 65, sunshineMinutes: 45),
+            Hour(17, temperature: 25, rain: 4, probability: 99, sunshineMinutes: 60));
+
+        var summary = SchoolDayWeatherSummary.From(snapshot, Day);
+
+        Assert.Equal(12, summary!.MinTemperatureC);
+        Assert.Equal(17, summary.MaxTemperatureC);
+        Assert.Equal(0, summary.PrecipitationMm);
+        Assert.Equal(65, summary.MaxHourlyPrecipitationProbabilityPercent);
+        Assert.Equal(1.3, summary.SunshineDurationHours);
+        Assert.Contains("08–16 Uhr", summary.Tooltip);
+        Assert.Contains("Regenwahrscheinlichkeit: bis 65 % (stündlich)", summary.Tooltip);
+        Assert.Contains($"Sonnenschein: {summary.SunshineDurationHours:0.#} h", summary.Tooltip);
+    }
+
+    [Fact]
+    public void From_AmtlicheGewitterwarnung_PraegtSymbolUndTooltip()
+    {
+        var snapshot = Snapshot(Hour(10, cloud: 20));
+        snapshot.Warnings.Add(new WeatherWarning
+        {
+            Identifier = "w1", Event = "GEWITTER", Headline = "Amtliche Warnung vor Gewitter",
+            Instruction = "Aufenthalt im Freien vermeiden.", Severity = "Severe",
+            Onset = Day.ToDateTime(new TimeOnly(12, 0)), Expires = Day.ToDateTime(new TimeOnly(16, 0)),
+        });
+
+        var summary = SchoolDayWeatherSummary.From(snapshot, Day);
+
+        Assert.Equal(SchoolDayWeatherKind.Thunderstorm, summary!.Kind);
+        Assert.Contains("Amtliche Warnung", summary.Tooltip);
+        Assert.Contains("Aufenthalt im Freien", summary.Tooltip);
+    }
+
+    [Fact]
+    public void From_RegenSummiertStundenUndZeigtTemperaturspanne()
+    {
+        var snapshot = Snapshot(
+            Hour(8, temperature: 12, rain: 0.1),
+            Hour(12, temperature: 17, rain: 0.4));
+
+        var summary = SchoolDayWeatherSummary.From(snapshot, Day);
+
+        Assert.Equal(SchoolDayWeatherKind.Rain, summary!.Kind);
+        Assert.Equal(0.5, summary.PrecipitationMm);
+        Assert.Contains("12–17 °C", summary.Tooltip);
+    }
+
+    [Fact]
+    public void WarningTouchesDate_MehrtaegigeVorwarnung_TrifftAlleUeberlapptenTage()
+    {
+        var warning = new WeatherWarning
+        {
+            Onset = Day.ToDateTime(new TimeOnly(18, 0)),
+            Expires = Day.AddDays(2).ToDateTime(new TimeOnly(8, 0)),
+        };
+
+        Assert.True(SchoolDayWeatherSummary.WarningTouchesDate(warning, Day.AddDays(1)));
+        Assert.False(SchoolDayWeatherSummary.WarningTouchesDate(warning, Day.AddDays(3)));
+    }
+
+    [Fact]
+    public void WeekCellItem_WetterOderWarnung_AktiviertGemeinsameHoverFlaeche()
+    {
+        var cell = WeekCellItem.WeekdayHeader(DayOfWeek.Monday, Day, false);
+        Assert.False(cell.HasWeatherInformation);
+
+        cell.SetWeather(SchoolDayWeatherSummary.From(Snapshot(Hour(10)), Day), []);
+        Assert.True(cell.HasWeatherInformation);
+
+        cell.SetWeather(null, [new WeatherWarning { Headline = "Amtliche Warnung" }]);
+        Assert.True(cell.HasWeatherInformation);
+        Assert.Contains("Amtliche Warnung", cell.WeatherTooltip);
+    }
+
+    private static WeatherSnapshot Snapshot(params WeatherForecastHour[] hours) => new()
+    {
+        RetrievedAt = DateTime.UtcNow,
+        Forecast = hours.ToList(),
+    };
+
+    private static WeatherForecastHour Hour(int hour, int? code = 0, double? temperature = 20,
+        double? rain = 0, double? gust = 20, double? cloud = 20,
+        double? probability = null, double? sunshineMinutes = null) => new()
+    {
+        ValidAt = Day.ToDateTime(new TimeOnly(hour, 0)), WeatherCode = code,
+        TemperatureC = temperature, PrecipitationMm = rain, WindGustKmh = gust,
+        CloudCoverPercent = cloud, PrecipitationProbabilityPercent = probability,
+        SunshineDurationMinutes = sunshineMinutes,
+    };
+}

LehrerApp.Desktop.Tests/SeatingPlanViewModelTests.cs

@@ -1,11 +1,41 @@
 using LehrerApp.Core.Models;
 using LehrerApp.Desktop.ViewModels.Groups;
+using LehrerApp.Desktop.Views.Groups;
 using Xunit;
 
 namespace LehrerApp.Desktop.Tests;
 
 public sealed class SeatingPlanViewModelTests
 {
+    [Fact]
+    public void TafelpositionWirdAusPlanGeladenUndBeimBearbeitenGespeichert()
+    {
+        var groupId = Guid.NewGuid();
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId,
+            Name = "Standard",
+            Rows = 1,
+            Columns = 1,
+            IsBoardAtBottom = true,
+        };
+        var plans = new FakeSeatingPlans([plan]);
+        var tabVm = new SeatingPlanTabViewModel(
+            plans, new FakeStudents([]), new FakeMemberships([]),
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+
+        tabVm.Initialize(groupId, isReadOnly: false);
+
+        Assert.True(tabVm.IsBoardAtBottom);
+        Assert.False(tabVm.IsBoardAtTop);
+
+        var dialogVm = tabVm.CreateDialogViewModel(plan);
+        dialogVm.IsBoardAtBottom = false;
+        dialogVm.SaveCommand.Execute(null);
+
+        Assert.False(plans.GetById(plan.Id)!.IsBoardAtBottom);
+    }
+
     [Fact]
     public void AuswahlEinesBereitsZugeordnetenSchuelers_VerschiebtIhnAufDenNeuenPlatz()
     {
@@ -24,8 +54,10 @@ public sealed class SeatingPlanViewModelTests
             Assignments = [new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id }],
         };
         var plans = new FakeSeatingPlans([plan]);
-        var vm = new SeatingPlanTabViewModel(plans, students, memberships);
+        var vm = new SeatingPlanTabViewModel(plans, students, memberships,
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
         vm.Initialize(groupId, isReadOnly: false);
+        vm.IsEditMode = true;
 
         vm.Seats[1].SelectedOption = vm.StudentOptions.Single(o => o.StudentId == student.Id);
 
@@ -40,7 +72,8 @@ public sealed class SeatingPlanViewModelTests
         var groupId = Guid.NewGuid();
         var plan = new SeatingPlan { GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1 };
         var vm = new SeatingPlanTabViewModel(
-            new FakeSeatingPlans([plan]), new FakeStudents([]), new FakeMemberships([]));
+            new FakeSeatingPlans([plan]), new FakeStudents([]), new FakeMemberships([]),
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
 
         vm.Initialize(groupId, isReadOnly: true);
 
@@ -48,4 +81,574 @@ public sealed class SeatingPlanViewModelTests
         Assert.False(vm.Seats.Single().CanEdit);
         Assert.False(vm.AddPlanCommand.CanExecute(null));
     }
+
+    [Fact]
+    public void DragDrop_ZwischenBelegtenPlaetzen_TauschtDieSchueler()
+    {
+        var groupId = Guid.NewGuid();
+        var anna = new Student { FirstName = "Anna", LastName = "A" };
+        var ben = new Student { FirstName = "Ben", LastName = "B" };
+        var students = new FakeStudents([anna, ben]);
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = groupId, StudentId = anna.Id },
+            new GroupMembership { GroupId = groupId, StudentId = ben.Id },
+        ]);
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 2,
+            Assignments =
+            [
+                new SeatAssignment { Row = 0, Column = 0, StudentId = anna.Id },
+                new SeatAssignment { Row = 0, Column = 1, StudentId = ben.Id },
+            ],
+        };
+        var plans = new FakeSeatingPlans([plan]);
+        var vm = new SeatingPlanTabViewModel(plans, students, memberships,
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+        vm.IsEditMode = true;
+
+        vm.MoveSeat(vm.Seats[0], vm.Seats[1]);
+
+        Assert.Equal(ben.Id, vm.Seats[0].SelectedOption.StudentId);
+        Assert.Equal(anna.Id, vm.Seats[1].SelectedOption.StudentId);
+        Assert.Equal(2, plans.GetById(plan.Id)!.Assignments.Count);
+    }
+
+    // ── Ausgeblendete Plätze (fehlender Tisch in einer Reihe) ─────────────────────────────
+
+    [Fact]
+    public void LoadPlan_LaedtAusgeblendetePlaetzeAusDemPlan()
+    {
+        var groupId = Guid.NewGuid();
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 2,
+            HiddenSeats = [new HiddenSeat { Row = 0, Column = 1 }],
+        };
+        var vm = new SeatingPlanTabViewModel(
+            new FakeSeatingPlans([plan]), new FakeStudents([]), new FakeMemberships([]),
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+
+        vm.Initialize(groupId, isReadOnly: false);
+
+        Assert.False(vm.Seats[0].IsHidden);
+        Assert.True(vm.Seats[1].IsHidden);
+    }
+
+    [Fact]
+    public void ToggleSeatHidden_LeererPlatz_BlendetAusUndSpeichertUndKannWiederEingeblendetWerden()
+    {
+        var groupId = Guid.NewGuid();
+        var plan = new SeatingPlan { GroupId = groupId, Name = "Standard", Rows = 1, Columns = 2 };
+        var plans = new FakeSeatingPlans([plan]);
+        var vm = new SeatingPlanTabViewModel(plans, new FakeStudents([]), new FakeMemberships([]),
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+        vm.IsEditMode = true;
+
+        vm.ToggleSeatHidden(vm.Seats[1]);
+
+        Assert.True(vm.Seats[1].IsHidden);
+        var hidden = Assert.Single(plans.GetById(plan.Id)!.HiddenSeats);
+        Assert.Equal((0, 1), (hidden.Row, hidden.Column));
+
+        vm.ToggleSeatHidden(vm.Seats[1]);
+
+        Assert.False(vm.Seats[1].IsHidden);
+        Assert.Empty(plans.GetById(plan.Id)!.HiddenSeats);
+    }
+
+    [Fact]
+    public void ToggleSeatHidden_BelegterPlatz_TutNichts()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = groupId, StudentId = student.Id },
+        ]);
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1,
+            Assignments = [new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id }],
+        };
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            memberships, new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+        vm.IsEditMode = true;
+
+        vm.ToggleSeatHidden(vm.Seats[0]);
+
+        Assert.False(vm.Seats[0].IsHidden);
+    }
+
+    /// Ein ausgeblendeter Platz darf nie ein Ziel für Drag&Drop/Zuordnung sein - sonst hätte man
+    /// gleichzeitig "kein Tisch hier" und einen dort sitzenden Schüler.
+    [Fact]
+    public void AssignStudent_AufAusgeblendetenPlatz_WirdIgnoriert()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = groupId, StudentId = student.Id },
+        ]);
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1,
+            HiddenSeats = [new HiddenSeat { Row = 0, Column = 0 }],
+        };
+        var plans = new FakeSeatingPlans([plan]);
+        var vm = new SeatingPlanTabViewModel(plans, new FakeStudents([student]), memberships,
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+        vm.IsEditMode = true;
+
+        vm.AssignStudent(vm.StudentOptions.Single(o => o.StudentId == student.Id), vm.Seats[0]);
+
+        Assert.False(vm.Seats[0].IsOccupied);
+        Assert.Empty(plans.GetById(plan.Id)!.Assignments);
+    }
+
+    [Fact]
+    public void MoveSeat_AufAusgeblendetenPlatz_WirdIgnoriert()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = groupId, StudentId = student.Id },
+        ]);
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 2,
+            HiddenSeats = [new HiddenSeat { Row = 0, Column = 1 }],
+            Assignments = [new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id }],
+        };
+        var plans = new FakeSeatingPlans([plan]);
+        var vm = new SeatingPlanTabViewModel(plans, new FakeStudents([student]), memberships,
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+        vm.IsEditMode = true;
+
+        vm.MoveSeat(vm.Seats[0], vm.Seats[1]);
+
+        Assert.Equal(student.Id, vm.Seats[0].SelectedOption.StudentId);
+        Assert.False(vm.Seats[1].IsOccupied);
+    }
+
+    // ── Unterrichtsmodus (14.x): Sitzung mit einer konkreten Stunde verknüpfen ────────────────
+
+    [Fact]
+    public void SelectOrCreateSessionForLesson_BereitsVerknuepfteSitzungVorhanden_WaehltSieAus()
+    {
+        var groupId = Guid.NewGuid();
+        var lesson = new Lesson { GroupId = groupId, Date = DateOnly.FromDateTime(DateTime.Today), Topic = "Redox" };
+        var existingSession = new ParticipationSession
+        { GroupId = groupId, Date = lesson.Date, LessonId = lesson.Id, Comment = "Redox" };
+        var sessions = new FakeSessions([existingSession]);
+        var plan = new SeatingPlan { GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1 };
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([]),
+            new FakeMemberships([]), sessions, new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+
+        vm.SelectOrCreateSessionForLesson(lesson);
+
+        Assert.Equal(existingSession.Id, vm.SelectedSession?.Id);
+        Assert.Single(sessions.GetByGroup(groupId));
+    }
+
+    [Fact]
+    public void SelectOrCreateSessionForLesson_KeineSitzungVorhanden_LegtVerknuepfteAnUndWaehltSieAus()
+    {
+        var groupId = Guid.NewGuid();
+        var lesson = new Lesson
+        { GroupId = groupId, Date = DateOnly.FromDateTime(DateTime.Today), Topic = "Redox" };
+        var sessions = new FakeSessions([]);
+        var plan = new SeatingPlan { GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1 };
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([]),
+            new FakeMemberships([]), sessions, new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+
+        vm.SelectOrCreateSessionForLesson(lesson);
+
+        var created = Assert.Single(sessions.GetByGroup(groupId));
+        Assert.Equal(lesson.Id, created.LessonId);
+        Assert.Equal(lesson.Topic, created.Comment);
+        Assert.Equal(vm.SelectedSession?.Id, created.Id);
+    }
+
+    [Fact]
+    public void SelectOrCreateSessionForLesson_ArchivierteGruppe_LegtKeineSitzungAn()
+    {
+        var groupId = Guid.NewGuid();
+        var lesson = new Lesson
+        { GroupId = groupId, Date = DateOnly.FromDateTime(DateTime.Today), Topic = "Redox" };
+        var sessions = new FakeSessions([]);
+        var plan = new SeatingPlan { GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1 };
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([]),
+            new FakeMemberships([]), sessions, new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: true);
+
+        vm.SelectOrCreateSessionForLesson(lesson);
+
+        Assert.Empty(sessions.GetByGroup(groupId));
+        Assert.Null(vm.SelectedSession);
+    }
+
+    [Fact]
+    public void ShowSeatUndCanToggleHidden_HaengenVonBearbeitungsmodusUndBelegungAb()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = groupId, StudentId = student.Id },
+        ]);
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 2,
+            HiddenSeats = [new HiddenSeat { Row = 0, Column = 1 }],
+            Assignments = [new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id }],
+        };
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            memberships, new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+
+        // Ansichtsmodus: der ausgeblendete Platz wird nicht gerendert, belegte Plätze können
+        // ohnehin nicht ausgeblendet werden.
+        Assert.False(vm.Seats[1].ShowSeat);
+        Assert.False(vm.Seats[0].CanToggleHidden);
+        Assert.False(vm.Seats[1].CanToggleHidden);
+
+        vm.IsEditMode = true;
+
+        // Bearbeitungsmodus: der ausgeblendete Platz bleibt sichtbar (abgeblendet), um ihn wieder
+        // einblenden zu können - der belegte Platz kann weiterhin nicht ausgeblendet werden.
+        Assert.True(vm.Seats[1].ShowSeat);
+        Assert.False(vm.Seats[0].CanToggleHidden);
+        Assert.True(vm.Seats[1].CanToggleHidden);
+    }
+
+    [Fact]
+    public void Ansichtsmodus_VerhindertVerschiebenUndBlendetBearbeitungsbefehleAus()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 2,
+            Assignments = [new SeatAssignment { Row = 0, Column = 0, StudentId = student.Id }],
+        };
+        var vm = new SeatingPlanTabViewModel(
+            new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            new FakeMemberships([new GroupMembership { GroupId = groupId, StudentId = student.Id }]),
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+
+        vm.MoveSeat(vm.Seats[0], vm.Seats[1]);
+
+        Assert.False(vm.IsEditMode);
+        Assert.False(vm.CanEditLayout);
+        Assert.False(vm.EditPlanCommand.CanExecute(null));
+        Assert.Equal(student.Id, vm.Seats[0].SelectedOption.StudentId);
+        Assert.Null(vm.Seats[1].SelectedOption.StudentId);
+
+        vm.IsEditMode = true;
+
+        Assert.True(vm.CanEditLayout);
+        Assert.True(vm.EditPlanCommand.CanExecute(null));
+        Assert.All(vm.Seats, seat => Assert.True(seat.CanEdit));
+    }
+
+    [Fact]
+    public void Ansichtsmodus_ZeigtAnwesenheitUndHausaufgabeUndBlendetAbwesendeAb()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var session = new ParticipationSession { GroupId = groupId, Date = DateOnly.FromDateTime(DateTime.Today) };
+        var entries = new FakeEntries();
+        entries.Add(new ParticipationEntry
+        {
+            SessionId = session.Id, StudentId = student.Id,
+            Attendance = AttendanceStatus.Excused, Homework = HomeworkStatus.MissingOpen,
+        });
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1,
+            Assignments = [new SeatAssignment { StudentId = student.Id }],
+        };
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            new FakeMemberships([new GroupMembership { GroupId = groupId, StudentId = student.Id }]),
+            new FakeSessions([session]), entries, new FakeAspects());
+
+        vm.Initialize(groupId, isReadOnly: false);
+
+        var seat = Assert.Single(vm.Seats);
+        Assert.True(seat.HasAttendanceBadge);
+        Assert.True(seat.HasHomeworkBadge);
+        Assert.Equal(0.42, seat.LessonOpacity);
+    }
+
+    [Fact]
+    public void SituationsTags_WerdenProSchuelerUndSitzungInEinemEntwurfZusammengefuehrt()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var session = new ParticipationSession
+        {
+            GroupId = groupId, Date = DateOnly.FromDateTime(DateTime.Today), LessonId = Guid.NewGuid(),
+        };
+        var docs = new FakeDocumentation();
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1,
+            Assignments = [new SeatAssignment { StudentId = student.Id }],
+        };
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            new FakeMemberships([new GroupMembership { GroupId = groupId, StudentId = student.Id }]),
+            new FakeSessions([session]), new FakeEntries(), new FakeAspects(), docs);
+        vm.Initialize(groupId, isReadOnly: false);
+        var seat = Assert.Single(vm.Seats);
+
+        seat.SituationTags.Single(t => t.Text == "Mitarbeit verweigert").ToggleCommand.Execute(null);
+        seat.SituationTags.Single(t => t.Text == "Material vergessen").ToggleCommand.Execute(null);
+
+        var draft = Assert.Single(docs.GetByStudent(student.Id));
+        Assert.True(draft.IsDraft);
+        Assert.Equal(session.Id, draft.ParticipationSessionId);
+        Assert.Equal(session.LessonId, draft.LessonId);
+        Assert.Equal(["Mitarbeit verweigert", "Material vergessen"], draft.Tags);
+    }
+
+    [Fact]
+    public void Tischabstaende_WerdenBeimBearbeitenProSpaltengrenzeGespeichert()
+    {
+        var groupId = Guid.NewGuid();
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 2, Columns = 4,
+            ColumnGapWidths = [0, 80, 20],
+        };
+        var plans = new FakeSeatingPlans([plan]);
+        var vm = new SeatingPlanDialogViewModel(plans, groupId, plan);
+
+        Assert.Equal([0m, 80m, 20m], vm.ColumnGaps.Select(g => g.Width));
+        vm.ColumnGaps[0].Width = 40;
+        vm.SaveCommand.Execute(null);
+
+        Assert.Equal([40d, 80d, 20d], plans.GetById(plan.Id)!.ColumnGapWidths);
+    }
+
+    [Fact]
+    public void SitzplanPanel_FuegtAbstandNurAnKonfigurierterSpaltengrenzeEin()
+    {
+        var panel = new SeatingPlanPanel
+        {
+            Columns = 3,
+            ColumnGapWidths = [0, 70],
+        };
+        for (var i = 0; i < 6; i++)
+            panel.Children.Add(new Avalonia.Controls.Border { Width = 100, Height = 50 });
+
+        panel.Measure(Avalonia.Size.Infinity);
+        panel.Arrange(new Avalonia.Rect(panel.DesiredSize));
+
+        Assert.Equal(370, panel.DesiredSize.Width);
+        Assert.Equal(100, panel.DesiredSize.Height);
+        Assert.Equal(0, panel.Children[0].Bounds.X);
+        Assert.Equal(100, panel.Children[1].Bounds.X);
+        Assert.Equal(270, panel.Children[2].Bounds.X);
+        Assert.Equal(270, panel.Children[5].Bounds.X);
+    }
+
+    [Fact]
+    public void SitzplatzBewertung_ErstelltHeutigeSitzungUndSpeichertAlleDreiBereiche()
+    {
+        var groupId = Guid.NewGuid();
+        var studentId = Guid.NewGuid();
+        var sessions = new FakeSessions([]);
+        var entries = new FakeEntries();
+        var vm = new SeatAssessmentViewModel(sessions, entries, new FakeAspects(),
+            groupId, studentId, "Beispiel, Anna", canEdit: true);
+
+        vm.SetRatingByNumber(5);
+        vm.ApplyAttendanceShortcut(1, clear: false);
+        vm.ApplyHomeworkShortcut(7, clear: false);
+
+        var session = Assert.Single(sessions.GetByGroup(groupId));
+        Assert.Equal(DateOnly.FromDateTime(DateTime.Today), session.Date);
+        Assert.Equal("Sitzplan", session.Comment);
+        var entry = entries.GetBySessionAndStudent(session.Id, studentId)!;
+        Assert.Equal(2, entry.Ratings.Single(r => r.Key == "quality").Value);
+        Assert.Equal(AttendanceStatus.Present, entry.Attendance);
+        Assert.Equal(HomeworkStatus.MissingOpen, entry.Homework);
+        Assert.True(entry.HomeworkMissing);
+    }
+
+    [Fact]
+    public void SitzplatzBewertung_ArchiviertOhneSitzung_LegtKeineNeueSitzungAn()
+    {
+        var groupId = Guid.NewGuid();
+        var sessions = new FakeSessions([]);
+
+        var vm = new SeatAssessmentViewModel(sessions, new FakeEntries(), new FakeAspects(),
+            groupId, Guid.NewGuid(), "Beispiel, Anna", canEdit: false);
+        vm.SetRatingByNumber(5);
+
+        Assert.False(vm.CanEdit);
+        Assert.Empty(sessions.GetByGroup(groupId));
+    }
+
+    [Fact]
+    public void Initialize_LegtOhneVorhandeneSitzungKeineGeistersitzungAn()
+    {
+        // Regressionstest: Initialize() läuft schon beim bloßen Navigieren zur Kursübersicht
+        // (GroupDetailViewModel.LoadGroup ruft SeatingPlanTab.Initialize für jede Gruppe auf),
+        // unabhängig davon, ob der Sitzplan-Tab überhaupt angesehen wird. Vorher legte
+        // LoadTodaySessions() dabei sofort eine "Sitzplan"-Sitzung für heute an.
+        var groupId = Guid.NewGuid();
+        var plan = new SeatingPlan { GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1 };
+        var sessions = new FakeSessions([]);
+        var vm = new SeatingPlanTabViewModel(
+            new FakeSeatingPlans([plan]), new FakeStudents([]), new FakeMemberships([]),
+            sessions, new FakeEntries(), new FakeAspects());
+
+        vm.Initialize(groupId, isReadOnly: false);
+
+        Assert.Empty(sessions.GetByGroup(groupId));
+        Assert.Null(vm.SelectedSession);
+    }
+
+    [Fact]
+    public async Task AssessStudent_LegtErstBeiTatsaechlicherBewertungEineSitzungAn()
+    {
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1,
+            Assignments = [new SeatAssignment { StudentId = student.Id }],
+        };
+        var sessions = new FakeSessions([]);
+        var vm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            new FakeMemberships([new GroupMembership { GroupId = groupId, StudentId = student.Id }]),
+            sessions, new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+        vm.OnAssessStudent = _ => Task.CompletedTask;
+        Assert.Empty(sessions.GetByGroup(groupId));
+
+        await vm.AssessStudent(vm.Seats.Single());
+
+        var session = Assert.Single(sessions.GetByGroup(groupId));
+        Assert.Equal(DateOnly.FromDateTime(DateTime.Today), session.Date);
+        Assert.Equal("Sitzplan", session.Comment);
+        Assert.Equal(session.Id, vm.SelectedSession?.Id);
+    }
+
+    [Fact]
+    public async Task AssessStudent_SitzungBereitsUeberAndereInstanzVerknuepft_LegtKeineZweiteAn()
+    {
+        // Regressionstest: Der Unterrichtsmodus läuft auf einer eigenen, per DI frisch aufgelösten
+        // SeatingPlanTabViewModel-Instanz (TimetableView.ShowTeachingMode), getrennt von der
+        // Instanz im Sitzplan-Tab der Kursübersicht — beide teilen sich nur das Repository. Wurde
+        // (wie hier simuliert) die Sitzung bereits über den Unterrichtsmodus mit der Stunde
+        // verknüpft, bevor im echten Sitzplan-Tab bewertet wird, darf letzterer keine zweite,
+        // unverknüpfte "Sitzplan"-Sitzung für denselben Tag anlegen.
+        var groupId = Guid.NewGuid();
+        var student = new Student { FirstName = "Anna", LastName = "A" };
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1,
+            Assignments = [new SeatAssignment { StudentId = student.Id }],
+        };
+        var sessions = new FakeSessions([]);
+        var memberships = new FakeMemberships([new GroupMembership { GroupId = groupId, StudentId = student.Id }]);
+
+        // Instanz A: Sitzplan-Tab der Kursübersicht, bereits initialisiert (kein SelectedSession).
+        var tabVm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            memberships, sessions, new FakeEntries(), new FakeAspects());
+        tabVm.Initialize(groupId, isReadOnly: false);
+        tabVm.OnAssessStudent = _ => Task.CompletedTask;
+
+        // Instanz B: eigene, frische Instanz wie im Unterrichtsmodus — verknüpft die Sitzung mit
+        // der Stunde, ohne dass Instanz A davon erfährt.
+        var lesson = new Lesson { GroupId = groupId, Date = DateOnly.FromDateTime(DateTime.Today), Topic = "Redox" };
+        var teachingModeVm = new SeatingPlanTabViewModel(new FakeSeatingPlans([plan]), new FakeStudents([student]),
+            memberships, sessions, new FakeEntries(), new FakeAspects());
+        teachingModeVm.Initialize(groupId, isReadOnly: false);
+        teachingModeVm.SelectOrCreateSessionForLesson(lesson);
+
+        var linked = Assert.Single(sessions.GetByGroup(groupId));
+        Assert.Equal(lesson.Id, linked.LessonId);
+
+        await tabVm.AssessStudent(tabVm.Seats.Single());
+
+        var session = Assert.Single(sessions.GetByGroup(groupId));
+        Assert.Equal(linked.Id, session.Id);
+        Assert.Equal(lesson.Id, session.LessonId);
+        Assert.Equal(session.Id, tabVm.SelectedSession?.Id);
+    }
+
+    [Fact]
+    public void ShuffleSeats_ErzeugtNeuenPlanMitVertauschtenPlaetzenUndBehaeltDenUrsprungsplan()
+    {
+        var groupId = Guid.NewGuid();
+        var anna = new Student { FirstName = "Anna", LastName = "A" };
+        var ben = new Student { FirstName = "Ben", LastName = "B" };
+        var cara = new Student { FirstName = "Cara", LastName = "C" };
+        var students = new FakeStudents([anna, ben, cara]);
+        var memberships = new FakeMemberships([
+            new GroupMembership { GroupId = groupId, StudentId = anna.Id },
+            new GroupMembership { GroupId = groupId, StudentId = ben.Id },
+            new GroupMembership { GroupId = groupId, StudentId = cara.Id },
+        ]);
+        var plan = new SeatingPlan
+        {
+            GroupId = groupId, Name = "Klausur", Room = "B204", Rows = 1, Columns = 3,
+            Assignments =
+            [
+                new SeatAssignment { Row = 0, Column = 0, StudentId = anna.Id },
+                new SeatAssignment { Row = 0, Column = 1, StudentId = ben.Id },
+                new SeatAssignment { Row = 0, Column = 2, StudentId = cara.Id },
+            ],
+        };
+        var plans = new FakeSeatingPlans([plan]);
+        var vm = new SeatingPlanTabViewModel(plans, students, memberships,
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+        vm.IsEditMode = true;
+
+        vm.ShuffleSeatsCommand.Execute(null);
+
+        var all = plans.GetByGroup(groupId);
+        Assert.Equal(2, all.Count);
+        var shuffled = Assert.Single(all, p => p.Id != plan.Id);
+        Assert.Equal("B204", shuffled.Room);
+        Assert.Equal(1, shuffled.Rows);
+        Assert.Equal(3, shuffled.Columns);
+        // Dieselben drei Schüler auf denselben drei Koordinaten, nur die Zuordnung untereinander
+        // wurde neu gewürfelt.
+        Assert.Equal(new[] { anna.Id, ben.Id, cara.Id }.OrderBy(id => id),
+            shuffled.Assignments.Select(a => a.StudentId).OrderBy(id => id));
+        Assert.Equal(new[] { (0, 0), (0, 1), (0, 2) },
+            shuffled.Assignments.Select(a => (a.Row, a.Column)).OrderBy(t => t.Item2));
+        // Ursprungsplan bleibt unverändert erhalten.
+        Assert.Equal(3, plans.GetById(plan.Id)!.Assignments.Count);
+        Assert.Equal(vm.SelectedPlan?.Id, shuffled.Id);
+    }
+
+    [Fact]
+    public void ShuffleSeatsCommand_OhneBearbeitungsmodus_NichtAusfuehrbar()
+    {
+        var groupId = Guid.NewGuid();
+        var plan = new SeatingPlan { GroupId = groupId, Name = "Standard", Rows = 1, Columns = 1 };
+        var vm = new SeatingPlanTabViewModel(
+            new FakeSeatingPlans([plan]), new FakeStudents([]), new FakeMemberships([]),
+            new FakeSessions([]), new FakeEntries(), new FakeAspects());
+        vm.Initialize(groupId, isReadOnly: false);
+
+        Assert.False(vm.ShuffleSeatsCommand.CanExecute(null));
+
+        vm.IsEditMode = true;
+
+        Assert.True(vm.ShuffleSeatsCommand.CanExecute(null));
+    }
 }

LehrerApp.Desktop.Tests/SettingsViewModelTests.cs

@@ -1,7 +1,11 @@
 using LehrerApp.Core.Models;
 using LehrerApp.Core.Services;
 using LehrerApp.Data;
+using LehrerApp.Desktop.Services;
 using LehrerApp.Desktop.ViewModels.Settings;
+using LehrerApp.Sync;
+using LehrerApp.Sync.Models;
+using System.Linq;
 using Xunit;
 
 namespace LehrerApp.Desktop.Tests;
@@ -10,7 +14,10 @@ public sealed class SettingsViewModelTests
 {
     private static SettingsViewModel BuildViewModel(FakeSchoolHolidays? holidays = null,
         FakeSupervisionDuties? supervisionDuties = null,
-        FakeSubjects? subjects = null, FakeCompetencyDomains? competencyDomains = null)
+        FakeSubjects? subjects = null, FakeCompetencyDomains? competencyDomains = null,
+        EventQueue? eventQueue = null, SyncKeyStatus? syncKeyStatus = null,
+        SyncKeyRecoveryService? syncKeyRecovery = null, AppearanceSettingsService? appearance = null,
+        TrashViewModel? trashTab = null)
     {
         // Bewusst kein "using": SchoolCalendarSettingsService liest den Pfad erst bei SetState,
         // das Verzeichnis muss über die Lebensdauer des ViewModels bestehen bleiben.
@@ -26,7 +33,188 @@ public sealed class SettingsViewModelTests
             new FakeDocumentation(), new FakeStudents([]), new FakeShorthandCodes([]),
             holidays ?? new FakeSchoolHolidays(), new SchoolCalendarSettingsService(tempPath),
             new PeriodScheduleService(tempPath), supervisionDuties ?? new FakeSupervisionDuties(),
-            new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService());
+            new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService(),
+            TestSupport.BuildWebUntisSettingsService(),
+            TestSupport.BuildAnnualPlanSettingsService(),
+            TestSupport.BuildSyncSettingsService(), TestSupport.BuildSyncAuthService(),
+            eventQueue ?? TestSupport.BuildEventQueue(), TestSupport.BuildAppLogger(),
+            syncKeyStatus ?? TestSupport.BuildSyncKeyStatus(),
+            syncKeyRecovery ?? TestSupport.BuildSyncKeyRecoveryService(),
+            appearance ?? TestSupport.BuildAppearanceSettingsService(),
+            trashTab ?? TestSupport.BuildTrashViewModel());
+    }
+
+    // ── Darstellung (12.4) ────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void SelectedTheme_SpiegeltDenGespeichertenStandBeimStartWider()
+    {
+        var appearance = TestSupport.BuildAppearanceSettingsService();
+        appearance.Save(AppTheme.Dark);
+
+        var vm = BuildViewModel(appearance: appearance);
+
+        Assert.Equal(AppTheme.Dark, vm.SelectedTheme);
+        Assert.Equal("Dunkel", vm.SelectedThemeName);
+    }
+
+    [Fact]
+    public void SelectedThemeName_Aendern_PersistiertUndBenachrichtigtDenCodeBehind()
+    {
+        var appearance = TestSupport.BuildAppearanceSettingsService();
+        var vm = BuildViewModel(appearance: appearance);
+        AppTheme? notified = null;
+        vm.OnThemeChanged = t => notified = t;
+
+        vm.SelectedThemeName = "Dunkel";
+
+        Assert.Equal(AppTheme.Dark, notified);
+        Assert.Equal(AppTheme.Dark, appearance.Load());
+    }
+
+    // ── Sync-Schlüssel: Warnung + Wiederherstellungscode (10.3.2) ────────────────────────────
+    //
+    // Der "glückliche" Redeem-Pfad (gültiger Code + Datei + Bestätigung) endet in
+    // AppBootstrapper.RestartApplication(), das Environment.Exit(0) aufruft - das darf ein Test
+    // niemals auslösen (würde den Testlauf beenden). Deshalb sind hier nur die Zweige getestet,
+    // die VOR diesem Aufruf zurückkehren; genau dasselbe Muster gilt bereits für
+    // RedeemPairingCode, das aus demselben Grund ebenfalls keinen Erfolgstest hat. Der
+    // eigentliche Kryptografie-/Datei-Roundtrip ist in SyncKeyRecoveryServiceTests abgedeckt.
+
+    [Fact]
+    public void SyncKeyWasRegenerated_SpiegeltDenBeimStartErkanntenStatusWider()
+    {
+        var vm = BuildViewModel(syncKeyStatus: new SyncKeyStatus(keyWasRegenerated: true));
+
+        Assert.True(vm.SyncKeyWasRegenerated);
+    }
+
+    [Fact]
+    public void SyncKeyWasRegenerated_NormalerStart_IstFalse()
+    {
+        var vm = BuildViewModel(syncKeyStatus: new SyncKeyStatus(keyWasRegenerated: false));
+
+        Assert.False(vm.SyncKeyWasRegenerated);
+    }
+
+    [Fact]
+    public async Task CreateRecoveryCode_SpeichernErfolgreich_ZeigtCodeUndUebergibtDateiinhalt()
+    {
+        var vm = BuildViewModel();
+        string? savedContent = null;
+        vm.OnSaveRecoveryFile = content => { savedContent = content; return Task.FromResult(true); };
+
+        await vm.CreateRecoveryCodeCommand.ExecuteAsync(null);
+
+        Assert.NotEmpty(vm.RecoveryCode);
+        Assert.NotNull(savedContent);
+        Assert.Contains("notieren", vm.RecoveryStatus);
+    }
+
+    [Fact]
+    public async Task CreateRecoveryCode_AbbruchBeimSpeichern_ZeigtKeinenCode()
+    {
+        var vm = BuildViewModel();
+        vm.OnSaveRecoveryFile = _ => Task.FromResult(false);
+
+        await vm.CreateRecoveryCodeCommand.ExecuteAsync(null);
+
+        Assert.Empty(vm.RecoveryCode);
+        Assert.Equal("Abgebrochen.", vm.RecoveryStatus);
+    }
+
+    [Fact]
+    public async Task RedeemRecoveryCode_LeererCode_FragtNieNachEinerDatei()
+    {
+        var vm = BuildViewModel();
+        var filePickerCalled = false;
+        vm.OnPickRecoveryFile = () => { filePickerCalled = true; return Task.FromResult<string?>(null); };
+        vm.RecoveryCodeInput = "   ";
+
+        await vm.RedeemRecoveryCodeCommand.ExecuteAsync(null);
+
+        Assert.False(filePickerCalled);
+        Assert.Equal("Bitte Code eingeben.", vm.RecoveryStatus);
+    }
+
+    [Fact]
+    public async Task RedeemRecoveryCode_DateiauswahlAbgebrochen_TutNichts()
+    {
+        var vm = BuildViewModel();
+        vm.OnPickRecoveryFile = () => Task.FromResult<string?>(null);
+        vm.RecoveryCodeInput = "IRGENDEIN-CODE";
+
+        var exception = await Record.ExceptionAsync(() => vm.RedeemRecoveryCodeCommand.ExecuteAsync(null));
+
+        Assert.Null(exception);
+    }
+
+    [Fact]
+    public async Task RedeemRecoveryCode_FalscherCode_ZeigtFehlerUndWirftNicht()
+    {
+        var recovery = TestSupport.BuildSyncKeyRecoveryService();
+        var vm = BuildViewModel(syncKeyRecovery: recovery);
+        vm.OnPickRecoveryFile = () => Task.FromResult<string?>(recovery.CreateRecoveryFile("RICHTIGER-CODE"));
+        vm.OnConfirmRecoveryRestore = () => Task.FromResult(true);
+        vm.RecoveryCodeInput = "FALSCHER-CODE";
+
+        var exception = await Record.ExceptionAsync(() => vm.RedeemRecoveryCodeCommand.ExecuteAsync(null));
+
+        Assert.Null(exception);
+        Assert.Contains("passen nicht zusammen", vm.RecoveryStatus);
+    }
+
+    [Fact]
+    public async Task RedeemRecoveryCode_BestaetigungAbgelehnt_TutNichts()
+    {
+        var recovery = TestSupport.BuildSyncKeyRecoveryService();
+        var vm = BuildViewModel(syncKeyRecovery: recovery);
+        vm.OnPickRecoveryFile = () => Task.FromResult<string?>(recovery.CreateRecoveryFile("EIN-CODE"));
+        vm.OnConfirmRecoveryRestore = () => Task.FromResult(false);
+        vm.RecoveryCodeInput = "EIN-CODE";
+
+        var exception = await Record.ExceptionAsync(() => vm.RedeemRecoveryCodeCommand.ExecuteAsync(null));
+
+        Assert.Null(exception);
+    }
+
+    [Fact]
+    public void SyncConflicts_ZeigtUnreviewedKonflikteBeimLaden()
+    {
+        var queue = TestSupport.BuildEventQueue();
+        var conflict = new ConflictEntry
+        {
+            LocalEvent = new SyncEvent { EntityType = "Student", EntityId = Guid.NewGuid().ToString(), Operation = "Save" },
+            RemoteEvent = new SyncEvent { EntityType = "Student", EntityId = Guid.NewGuid().ToString(), Operation = "Save" },
+            Resolution = "RemoteWon",
+        };
+        queue.AddConflict(conflict);
+
+        var vm = BuildViewModel(eventQueue: queue);
+
+        var item = Assert.Single(vm.SyncConflicts);
+        Assert.Equal(conflict.Id, item.Id);
+        Assert.Contains("anderen Gerät", item.ResolutionDisplay);
+    }
+
+    [Fact]
+    public void MarkConflictReviewed_EntferntKonfliktAusListeUndAusDerQueue()
+    {
+        var queue = TestSupport.BuildEventQueue();
+        var conflict = new ConflictEntry
+        {
+            LocalEvent = new SyncEvent { EntityType = "Student", EntityId = Guid.NewGuid().ToString(), Operation = "Save" },
+            RemoteEvent = new SyncEvent { EntityType = "Student", EntityId = Guid.NewGuid().ToString(), Operation = "Save" },
+            Resolution = "LocalWon",
+        };
+        queue.AddConflict(conflict);
+        var vm = BuildViewModel(eventQueue: queue);
+        var item = vm.SyncConflicts.Single();
+
+        vm.MarkConflictReviewedCommand.Execute(item);
+
+        Assert.Empty(vm.SyncConflicts);
+        Assert.Empty(queue.GetUnreviewed());
     }
 
     [Fact]
@@ -87,6 +275,34 @@ public sealed class SettingsViewModelTests
         Assert.Empty(vm.SchoolHolidayEntries);
     }
 
+    [Fact]
+    public async Task SaveSchoolLocation_FehlendeFelder_ZeigtAlleFeldfehler()
+    {
+        var vm = BuildViewModel();
+
+        await vm.SaveSchoolLocationCommand.ExecuteAsync(null);
+
+        Assert.NotEmpty(vm.SchoolNameError);
+        Assert.NotEmpty(vm.SchoolStreetError);
+        Assert.NotEmpty(vm.SchoolPostalCodeError);
+        Assert.NotEmpty(vm.SchoolCityError);
+    }
+
+    [Fact]
+    public async Task SaveSchoolLocation_UngueltigePlz_RuftKeinenServerAuf()
+    {
+        var vm = BuildViewModel();
+        vm.SchoolName = "Testschule";
+        vm.SchoolStreet = "Schulweg 1";
+        vm.SchoolPostalCode = "1234";
+        vm.SchoolCity = "Teststadt";
+
+        await vm.SaveSchoolLocationCommand.ExecuteAsync(null);
+
+        Assert.Contains("fünfstellige", vm.SchoolPostalCodeError);
+        Assert.Empty(vm.SchoolLocationStatus);
+    }
+
     [Fact]
     public void SelectedStateName_Aendern_PersistiertUeberSchoolCalendarSettings()
     {
@@ -102,7 +318,12 @@ public sealed class SettingsViewModelTests
             new LiteDbContext(new MemoryStream()), new PrivacySettingsService(tempPath),
             new FakeDocumentation(), new FakeStudents([]), new FakeShorthandCodes([]),
             new FakeSchoolHolidays(), calendarSettings, new PeriodScheduleService(tempPath),
-            new FakeSupervisionDuties(), new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService());
+            new FakeSupervisionDuties(), new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService(),
+            TestSupport.BuildWebUntisSettingsService(),
+            TestSupport.BuildAnnualPlanSettingsService(),
+            TestSupport.BuildSyncSettingsService(), TestSupport.BuildSyncAuthService(), TestSupport.BuildEventQueue(),
+            TestSupport.BuildAppLogger(), TestSupport.BuildSyncKeyStatus(), TestSupport.BuildSyncKeyRecoveryService(),
+            TestSupport.BuildAppearanceSettingsService(), TestSupport.BuildTrashViewModel());
 
         vm.SelectedStateName = "Bayern";
 
@@ -124,7 +345,12 @@ public sealed class SettingsViewModelTests
             new LiteDbContext(new MemoryStream()), new PrivacySettingsService(tempPath),
             new FakeDocumentation(), new FakeStudents([]), new FakeShorthandCodes([]),
             new FakeSchoolHolidays(), new SchoolCalendarSettingsService(tempPath), periodSchedule,
-            new FakeSupervisionDuties(), new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService());
+            new FakeSupervisionDuties(), new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService(),
+            TestSupport.BuildWebUntisSettingsService(),
+            TestSupport.BuildAnnualPlanSettingsService(),
+            TestSupport.BuildSyncSettingsService(), TestSupport.BuildSyncAuthService(), TestSupport.BuildEventQueue(),
+            TestSupport.BuildAppLogger(), TestSupport.BuildSyncKeyStatus(), TestSupport.BuildSyncKeyRecoveryService(),
+            TestSupport.BuildAppearanceSettingsService(), TestSupport.BuildTrashViewModel());
 
         vm.PeriodTimes[0].StartText = "08:00";
         vm.PeriodTimes[0].EndText = "08:45";
@@ -150,7 +376,12 @@ public sealed class SettingsViewModelTests
             new LiteDbContext(new MemoryStream()), new PrivacySettingsService(tempPath),
             new FakeDocumentation(), new FakeStudents([]), new FakeShorthandCodes([]),
             new FakeSchoolHolidays(), new SchoolCalendarSettingsService(tempPath), periodSchedule,
-            new FakeSupervisionDuties(), new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService());
+            new FakeSupervisionDuties(), new LetterTemplateService(tempPath), TestSupport.BuildAiSettingsService(), TestSupport.BuildAiPlanningService(),
+            TestSupport.BuildWebUntisSettingsService(),
+            TestSupport.BuildAnnualPlanSettingsService(),
+            TestSupport.BuildSyncSettingsService(), TestSupport.BuildSyncAuthService(), TestSupport.BuildEventQueue(),
+            TestSupport.BuildAppLogger(), TestSupport.BuildSyncKeyStatus(), TestSupport.BuildSyncKeyRecoveryService(),
+            TestSupport.BuildAppearanceSettingsService(), TestSupport.BuildTrashViewModel());
 
         vm.PeriodTimes[0].StartText = "08:45";
         vm.PeriodTimes[0].EndText = "08:00";

LehrerApp.Desktop.Tests/StudentImportDialogViewModelTests.cs

@@ -0,0 +1,116 @@
+using LehrerApp.Core.Importing;
+using LehrerApp.Core.Models;
+using LehrerApp.Core.Services;
+using LehrerApp.Desktop.ViewModels.Students;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class StudentImportDialogViewModelTests
+{
+    [Fact]
+    public async Task Vorschau_ZeigtZusammenfassungUndImportiertNeueSchueler()
+    {
+        var stored = new List<Student>();
+        var service = Service(stored,
+        [
+            new ImportedStudent
+            {
+                FirstName = "Max",
+                LastName = "Beispiel",
+                Gender = Gender.M,
+            },
+        ]);
+        var preview = await service.AnalyzeAsync(File());
+        var vm = new StudentImportDialogViewModel(service, preview, "students.csv");
+
+        Assert.Contains("1 erkannt", vm.Summary);
+        Assert.Equal(1, vm.NewStudents);
+        Assert.True(vm.IsReadyWithoutConflicts);
+
+        Assert.True(await vm.TryApplyAsync());
+        Assert.Equal(1, vm.Result?.CreatedStudents);
+        Assert.Single(stored);
+    }
+
+    [Fact]
+    public async Task Konflikt_VerwendetDieSichereStandardoptionUeberspringen()
+    {
+        var stored = new List<Student>
+        {
+            new() { FirstName = "Max", LastName = "Beispiel" },
+        };
+        var service = Service(stored,
+        [
+            new ImportedStudent { FirstName = "Max", LastName = "Beispiel" },
+        ]);
+        var preview = await service.AnalyzeAsync(File());
+        var vm = new StudentImportDialogViewModel(service, preview, "students.csv");
+
+        var conflict = Assert.Single(vm.Conflicts);
+        Assert.Equal("skip", conflict.SelectedOption.Id);
+        Assert.Equal("skip", Assert.Single(vm.BuildDecisions()).SelectedOptionId);
+
+        Assert.True(await vm.TryApplyAsync());
+        Assert.Equal(1, vm.Result?.SkippedStudents);
+        Assert.Single(stored);
+    }
+
+    [Fact]
+    public async Task UebernehmenFuerAlle_WaehltBeiJedemEindeutigenKonfliktDenVorhandenenSchueler()
+    {
+        var stored = new List<Student>
+        {
+            new() { FirstName = "Max", LastName = "Beispiel" },
+            new() { FirstName = "Erika", LastName = "Muster" },
+        };
+        var service = Service(stored,
+        [
+            new ImportedStudent { FirstName = "Max", LastName = "Beispiel" },
+            new ImportedStudent { FirstName = "Erika", LastName = "Muster" },
+        ]);
+        var preview = await service.AnalyzeAsync(File());
+        var vm = new StudentImportDialogViewModel(service, preview, "students.csv");
+
+        Assert.True(vm.CanUseExistingForAll);
+        Assert.All(vm.Conflicts, conflict => Assert.Equal("skip", conflict.SelectedOption.Id));
+
+        vm.UseExistingForAll();
+
+        Assert.All(vm.Conflicts,
+            conflict => Assert.StartsWith("existing:", conflict.SelectedOption.Id));
+        Assert.True(await vm.TryApplyAsync());
+        Assert.Equal(2, vm.Result?.MatchedExistingStudents);
+        Assert.Equal(0, vm.Result?.SkippedStudents);
+        Assert.Equal(2, stored.Count);
+    }
+
+    private static StudentImportService Service(
+        List<Student> stored,
+        IReadOnlyList<ImportedStudent> imported) => new(
+        [new FakeImportHandler(imported)],
+        new FakeStudents(stored),
+        new FakeGroups([]),
+        new FakeMemberships([]),
+        new SchoolYearService());
+
+    private static ImportFile File() => new("students.csv", "test"u8.ToArray());
+
+    private sealed class FakeImportHandler(IReadOnlyList<ImportedStudent> imported)
+        : IImportHandler<ImportedStudent>
+    {
+        public ImportFormatId FormatId { get; } = new("test.students");
+        public string DisplayName => "Testformat";
+        public IReadOnlyCollection<string> SupportedExtensions { get; } = [".csv"];
+
+        public ValueTask<ImportDetection> DetectAsync(
+            ImportFile file,
+            CancellationToken cancellationToken = default) =>
+            ValueTask.FromResult(ImportDetection.Match(100));
+
+        public ValueTask<ImportParseResult<ImportedStudent>> ParseAsync(
+            ImportFile file,
+            CancellationToken cancellationToken = default) =>
+            ValueTask.FromResult(new ImportParseResult<ImportedStudent>(imported));
+    }
+}

LehrerApp.Desktop.Tests/SyncAuthServiceProtocolTests.cs

@@ -0,0 +1,33 @@
+using System.Net;
+using LehrerApp.Desktop.Services;
+using LehrerApp.Sync;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class SyncAuthServiceProtocolTests
+{
+    [Fact]
+    public async Task TestConnectionAsync_InkompatibleVersion_WirdNichtAlsOfflineGemeldet()
+    {
+        HttpRequestMessage? sent = null;
+        var service = new SyncAuthService(new HttpClient(new Handler(request =>
+        {
+            sent = request;
+            return new HttpResponseMessage(HttpStatusCode.UpgradeRequired);
+        })));
+
+        var result = await service.TestConnectionAsync("https://sync.example.invalid", "token");
+
+        Assert.Equal(SyncConnectionTestResult.IncompatibleVersion, result);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            sent!.Headers.GetValues(SyncProtocol.VersionHeaderName).Single());
+    }
+
+    private sealed class Handler(Func<HttpRequestMessage, HttpResponseMessage> response) : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request, CancellationToken cancellationToken) =>
+            Task.FromResult(response(request));
+    }
+}

LehrerApp.Desktop.Tests/SyncSettingsServiceTests.cs

@@ -0,0 +1,98 @@
+using LehrerApp.Desktop.Services;
+using Xunit;
+
+namespace LehrerApp.Desktop.Tests;
+
+public sealed class SyncSettingsServiceTests
+{
+    private static string BuildTempPath()
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"lehrerapp-syncsettingssvc-tests-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(path);
+        return path;
+    }
+
+    [Fact]
+    public void SetServerUrl_PersistiertUeberNeueInstanz()
+    {
+        var path = BuildTempPath();
+        new SyncSettingsService(path).SetServerUrl("https://sync.example.com");
+
+        var reloaded = new SyncSettingsService(path);
+
+        Assert.Equal("https://sync.example.com", reloaded.ServerUrl);
+    }
+
+    [Fact]
+    public void SetCredentialsAndToken_TokenIstVerschluesseltAbrufbar()
+    {
+        var service = new SyncSettingsService(BuildTempPath());
+
+        service.SetCredentialsAndToken("sebastian", "geheimes-token-123");
+
+        Assert.True(service.IsLoggedIn);
+        Assert.Equal("sebastian", service.Username);
+        Assert.Equal("geheimes-token-123", service.GetToken());
+    }
+
+    [Fact]
+    public void Token_UeberlebtNeueInstanzMitDemselbenPfad()
+    {
+        var path = BuildTempPath();
+        new SyncSettingsService(path).SetCredentialsAndToken("sebastian", "token-abc");
+
+        var reloaded = new SyncSettingsService(path);
+
+        Assert.True(reloaded.IsLoggedIn);
+        Assert.Equal("token-abc", reloaded.GetToken());
+    }
+
+    [Fact]
+    public void TokenDateiEnthaeltNichtDenKlartext()
+    {
+        var path = BuildTempPath();
+        var service = new SyncSettingsService(path);
+        service.SetCredentialsAndToken("sebastian", "geheimes-token-123");
+
+        var raw = File.ReadAllText(Path.Combine(path, "sync-settings.json"));
+
+        Assert.DoesNotContain("geheimes-token-123", raw);
+    }
+
+    [Fact]
+    public void SetCredentialsAndToken_MitUserId_PersistiertLastUserIdUeberNeueInstanz()
+    {
+        var path = BuildTempPath();
+        new SyncSettingsService(path).SetCredentialsAndToken("Sebastian", "token-abc", "sebastian");
+
+        var reloaded = new SyncSettingsService(path);
+
+        Assert.Equal("sebastian", reloaded.LastUserId);
+    }
+
+    [Fact]
+    public void SetCredentialsAndToken_OhneUserId_LaesstVorhandeneLastUserIdUnveraendert()
+    {
+        var service = new SyncSettingsService(BuildTempPath());
+        service.SetCredentialsAndToken("sebastian", "token-1", "sebastian");
+
+        service.SetCredentialsAndToken("sebastian", "token-2");
+
+        Assert.Equal("sebastian", service.LastUserId);
+    }
+
+    [Fact]
+    public void Logout_EntferntTokenBehaeltAberServerUrlUndUsername()
+    {
+        var service = new SyncSettingsService(BuildTempPath());
+        service.SetServerUrl("https://sync.example.com");
+        service.SetCredentialsAndToken("sebastian", "token-abc");
+
+        service.Logout();
+
+        Assert.False(service.IsLoggedIn);
+        Assert.Null(service.GetToken());
+        Assert.Equal("https://sync.example.com", service.ServerUrl);
+        Assert.Equal("sebastian", service.Username);
+    }
+}