feat: sync guard - Blockieren alter Clients beim sync

LehrerApp.Sync.Tests/AttachmentSyncerTests.cs

@@ -34,6 +34,8 @@ public sealed class AttachmentSyncerTests
         var request = Assert.Single(handler.Requests);
         Assert.Equal(HttpMethod.Post, request.Method);
         Assert.Equal($"/api/sync/attachments/{storageId}", request.RequestUri!.AbsolutePath);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            request.Headers.GetValues(SyncProtocol.VersionHeaderName).Single());
         Assert.NotNull(uploadedEncrypted);
         Assert.Equal([1, 2, 3, 4], SyncCrypto.Decrypt(uploadedEncrypted!, Key));
         Assert.Empty(temp.Queue.GetPendingAttachmentUploads());

LehrerApp.Sync.Tests/SyncEngineTests.cs

@@ -13,6 +13,33 @@ public sealed class SyncEngineTests
     private static LiteDbContext NewInMemoryContext() => new(new MemoryStream());
     private static readonly byte[] Key = SyncCrypto.GenerateKey();
 
+    [Fact]
+    public async Task SyncNowAsync_InkompatibleServerVersion_BrichtVorWeiterenRequestsAb()
+    {
+        using var temp = new TempEventQueue();
+        var pending = temp.Queue.Enqueue("this-device", DeviceType.Desktop,
+            "Lesson", Guid.NewGuid().ToString(), "Save", "lokale-daten");
+        var handler = new FakeHttpMessageHandler(req =>
+        {
+            var response = new HttpResponseMessage(HttpStatusCode.UpgradeRequired);
+            response.Headers.Add(SyncProtocol.VersionHeaderName, "2");
+            return response;
+        });
+        var engine = MakeEngine(temp, handler);
+
+        var result = await engine.SyncNowAsync();
+
+        Assert.False(result.Success);
+        Assert.Contains("Sync-Version 1", result.Reason);
+        Assert.Contains("Server Version 2", result.Reason);
+        Assert.Equal(SyncState.IncompatibleVersion, engine.Status.State);
+        Assert.Equal(pending.EventId, Assert.Single(temp.Queue.GetPending()).EventId);
+        var request = Assert.Single(handler.Requests);
+        Assert.Equal("/api/sync/push", request.RequestUri!.AbsolutePath);
+        Assert.Equal(SyncProtocol.CurrentVersion,
+            request.Headers.GetValues(SyncProtocol.VersionHeaderName).Single());
+    }
+
     /// Regression: EventApplier.ApplyAsync fing früher nur LiteException ab. Jede andere Ausnahme
     /// (z.B. eine ungültige/korrupte Payload eines einzelnen Ereignisses) fiel unbehandelt aus
     /// SyncEngine.PullAsync heraus, BEVOR _queue.SetLastServerSeq() erreicht wurde — der nächste