feat: Anhänge je Stunde, Klausur-Sitzplan mischen, Gefährdungsbeurteilungs-Assistent

Lesson bekommt dieselbe Anhang-Infrastruktur wie Documentation (Material,
Arbeitsblätter, Experimentunterlagen), samt Fix einer Sync-Lücke, die Anhang-
Dateibytes bisher nur für Documentation statt generisch übertragen hat
(IHasAttachments). Sitzplan-Tab bekommt einen "Plätze mischen"-Button für
Klausursitzpläne. Neu: mehrschrittiger Gefährdungsbeurteilungs-Assistent mit
optionalem KI-Entwurf (ai-backend/gbu.php) und PDF-Export, Format bewusst als
JSON-Anhang statt eigener Datenbank-Entität. Details und Architekturentscheidungen
in TODO.md (4.2, 7.1.5, 10.1.8).

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

LehrerApp.Desktop.Tests/SeatingPlanViewModelTests.cs

@@ -542,4 +542,69 @@ public sealed class SeatingPlanViewModelTests
         Assert.Equal("Sitzplan", session.Comment);
         Assert.Equal(session.Id, vm.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));
+    }
 }