feat(libraries): honest scan lifecycle + one-lock-one-release discipline (#232)

Scan queue handler now returns a QueueLibraryScanResult enum
(Queued|NotFound|SyncDisabled|AlreadyScanning) instead of a lying bool;
LibrariesController.ScanLibrary maps them to 202/404/422/409 with ProblemDetails.
Guard the lock->enqueue with the EnqueueWithTraktLock compensating-unlock pattern.

ScannerService now releases every library/collection lock in a finally so a handler
exception can't leak the lock. Plex "Shows" scheduler batch (one lock, two messages)
now has only the trailing SynchronizePlexNetworks carry the single release
(Unlock flag), mirroring the scheduler Trakt tail-token precedent.

Guard the other lock->enqueue producers (Create/UpdateLocalLibrary, UpdateTraktList)
with compensating unlock. SPA drops the PENDING_GRACE_TICKS heuristic now that the
POST reports 202/409/404/422 directly: 202 -> pending+poll, 409 -> reconcile (no
error toast), 404/422 -> surface error.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV/Controllers/Api/LibrariesController.cs

@@ -22,10 +22,29 @@ public class LibrariesController(ITelevisionRepository televisionRepository, IMe
     [HttpPost("/api/libraries/{id:int}/scan")]
     [Tags("Libraries")]
     [EndpointSummary("Scan library")]
-    public async Task<IActionResult> ScanLibrary(int id) =>
-        await mediator.Send(new QueueLibraryScanByLibraryId(id))
-            ? new OkResult()
-            : new NotFoundResult();
+    [ProducesResponseType(StatusCodes.Status202Accepted)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status404NotFound)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status409Conflict)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status422UnprocessableEntity)]
+    public async Task<IActionResult> ScanLibrary(int id, CancellationToken cancellationToken)
+    {
+        QueueLibraryScanResult result = await mediator.Send(new QueueLibraryScanByLibraryId(id), cancellationToken);
+        return result switch
+        {
+            QueueLibraryScanResult.Queued => new AcceptedResult(),
+            QueueLibraryScanResult.AlreadyScanning => ApiResults.ConflictProblem(
+                "Library scan in progress",
+                $"A scan for library {id} is already in progress."),
+            QueueLibraryScanResult.SyncDisabled => new UnprocessableEntityObjectResult(
+                new ProblemDetails
+                {
+                    Status = StatusCodes.Status422UnprocessableEntity,
+                    Title = "Library sync is disabled",
+                    Detail = $"Item sync is disabled for library {id}."
+                }),
+            _ => ApiResults.NotFoundProblem($"Library {id} does not exist.")
+        };
+    }
 
     [HttpPost("/api/libraries/{id:int}/scan-show")]
     [Tags("Libraries")]