fix: address #167 review — release Trakt lock on failed enqueue; mark busy after generatePlaylist save

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

ErsatzTV/Controllers/Api/TraktController.cs

@@ -76,13 +76,7 @@ public partial class TraktController(
                 CreateProblemDetails(422, "Validation failed", "Invalid Trakt list url"));
         }
 
-        if (!entityLocker.LockTrakt())
-        {
-            return ConflictProblem();
-        }
-
-        await workerChannel.WriteAsync(AddTraktList.FromUrl(request.Url), cancellationToken);
-        return new AcceptedResult();
+        return await EnqueueWithTraktLock(AddTraktList.FromUrl(request.Url), cancellationToken);
     }
 
     [HttpPost("/api/trakt/lists/{id:int}/match")]
@@ -100,13 +94,7 @@ public partial class TraktController(
             return ApiResults.NotFoundProblem();
         }
 
-        if (!entityLocker.LockTrakt())
-        {
-            return ConflictProblem();
-        }
-
-        await workerChannel.WriteAsync(new MatchTraktListItems(id), cancellationToken);
-        return new AcceptedResult();
+        return await EnqueueWithTraktLock(new MatchTraktListItems(id), cancellationToken);
     }
 
     [HttpDelete("/api/trakt/lists/{id:int}")]
@@ -124,13 +112,7 @@ public partial class TraktController(
             return ApiResults.NotFoundProblem();
         }
 
-        if (!entityLocker.LockTrakt())
-        {
-            return ConflictProblem();
-        }
-
-        await workerChannel.WriteAsync(new DeleteTraktList(id), cancellationToken);
-        return new AcceptedResult();
+        return await EnqueueWithTraktLock(new DeleteTraktList(id), cancellationToken);
     }
 
     [HttpPut("/api/trakt/lists/{id:int}")]
@@ -185,6 +167,30 @@ public partial class TraktController(
             viewModel.AutoRefresh,
             viewModel.GeneratePlaylist);
 
+    private async Task<IActionResult> EnqueueWithTraktLock(
+        IBackgroundServiceRequest request,
+        CancellationToken cancellationToken)
+    {
+        if (!entityLocker.LockTrakt())
+        {
+            return ConflictProblem();
+        }
+
+        try
+        {
+            await workerChannel.WriteAsync(request, cancellationToken);
+        }
+        catch
+        {
+            // the background handler only unlocks when it receives the message;
+            // if enqueueing fails (e.g. request aborted), release the lock here or it is held forever
+            entityLocker.UnlockTrakt();
+            throw;
+        }
+
+        return new AcceptedResult();
+    }
+
     private static ConflictObjectResult ConflictProblem() =>
         new ConflictObjectResult(
             CreateProblemDetails(