Fix Critical: IndexSystem lock races and TrackRepository.Update silent-create

- Replace object lock with SemaphoreSlim(1,1) on both DirectoryIndexDirectory
  and VaultIndexDirectory; SaveIndexAsync now executes inside the semaphore
  so mutate+persist is atomic
- ReloadIndexAsync acquires the semaphore before LoadIndexAsync so disk load
  and in-memory swap are atomic with respect to concurrent writes
- HasIndexEntry and GetEntryMetadata converted to async Task with WaitAsync;
  MediaVault.GetEntryAsync call sites updated accordingly
- TrackRepository.Update throws InvalidOperationException when Id not found
  instead of silently calling Create; service layer catches and wraps as fail result

DeepDrftWeb.Services/Repositories/TrackRepository.cs

@@ -50,7 +50,7 @@ public class TrackRepository
 
         if (trackEntity == null)
         {
-            return await Create(track);
+            throw new InvalidOperationException($"Track not found: {track.Id}");
         }
         
         trackEntity.Album = track.Album;