feat(235): F9 API parity — library deep-scan, external-collections scan, scan-show outcome enum (#235 slice B)

Closes the two F9 Libraries.razor parity gaps and normalizes scan-show error
mapping to ProblemDetails.

TASK 1 — library-wide deep scan:
- QueueLibraryScanByLibraryId gains optional `bool DeepScan = false`; handler
  threads it into ForceSynchronize{Plex,Jellyfin,Emby}LibraryById.
- POST /api/libraries/{id}/scan?deep=false binds it via [FromQuery].

TASK 2 — external-collections scan (new endpoints):
- POST /api/media-sources/{plex|jellyfin|emby}/{id}/scan-collections?deep=false
  acquires the per-source collections lock (§3b: lock IS the running scan → 409),
  enqueues Synchronize{X}Collections(id, ForceScan:true, deep) to the scanner
  channel, returns 202; compensating-unlock on enqueue throw.

TASK 3 — scan-show normalization:
- New QueueShowScanResult enum; handler returns it instead of bool.
- POST /api/libraries/{id}/scan-show now maps 202/404/409/422 (all errors
  ProblemDetails) instead of 200/404/400-anonymous-object.
- Updated the lone Blazor caller (TelevisionSeasonList.razor).

Tests: LibrariesController (scan deep=true, scan-show enum→status), the three
media-source controllers (scan-collections route/404/409/202/compensating-unlock),
and handler tests for both changed handlers (deep threading + show-scan outcomes).
Docs: api-conventions §3b exemplar + blazor-route-parity §5 F9 gate.

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

ErsatzTV/Controllers/Api/JellyfinMediaSourcesController.cs

@@ -278,6 +278,53 @@ public class JellyfinMediaSourcesController(
         return new AcceptedResult();
     }
 
+    [HttpPost("/api/media-sources/jellyfin/{id:int}/scan-collections", Name = "ScanJellyfinCollections")]
+    [Tags("Jellyfin")]
+    [EndpointSummary("Scan a Jellyfin source's collections")]
+    [EndpointDescription(
+        "Queues a synchronization of the source's collections (fire-and-forget). Pass ?deep=true for a deep " +
+        "scan. Returns 409 while a Jellyfin collections scan is already in progress.")]
+    [EndpointGroupName("general")]
+    [ProducesResponseType(StatusCodes.Status202Accepted)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status404NotFound)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status409Conflict)]
+    public async Task<IActionResult> ScanCollections(
+        int id,
+        [FromQuery] bool deep = false,
+        CancellationToken cancellationToken = default)
+    {
+        Option<JellyfinMediaSourceViewModel> maybeSource =
+            await mediator.Send(new GetJellyfinMediaSourceById(id), cancellationToken);
+        if (maybeSource.IsNone)
+        {
+            return ApiResults.NotFoundProblem();
+        }
+
+        // The collections lock IS the running scan (§3b): fail to acquire = a scan is already active → 409.
+        if (!entityLocker.LockJellyfinCollections())
+        {
+            return ApiResults.ConflictProblem(
+                "Jellyfin collections scan in progress",
+                "A Jellyfin collections scan is already in progress; try again once it completes.");
+        }
+
+        try
+        {
+            await scannerWorkerChannel.WriteAsync(
+                new SynchronizeJellyfinCollections(id, true, deep),
+                cancellationToken);
+        }
+        catch
+        {
+            // the scanner releases the lock when it processes the message; if the enqueue throws after we
+            // acquired the lock, release it here (EnqueueWithTraktLock compensating-unlock, §3b)
+            entityLocker.UnlockJellyfinCollections();
+            throw;
+        }
+
+        return new AcceptedResult();
+    }
+
     // §C7: LockLibrary then enqueue the sync pair; a throw from the enqueue compensates by unlocking
     // (EnqueueWithTraktLock pattern) — a locked library is silently skipped (Blazor parity).
     private async Task EnqueueLibrarySync(int sourceId, int libraryId, CancellationToken cancellationToken)