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

DeepDrftContent.Services/FileDatabase/Services/MediaVault.cs

@@ -64,11 +64,11 @@ public abstract class MediaVault : VaultIndexDirectory
         var vaultType = MediaVaultTypeMap.GetVaultType<T>();
 
         // Use thread-safe method from VaultIndexDirectory
-        if (!HasIndexEntry(entryId))
+        if (!await HasIndexEntry(entryId))
             return null;
 
         // Use thread-safe metadata retrieval
-        var metaData = GetEntryMetadata(entryId);
+        var metaData = await GetEntryMetadata(entryId);
         if (metaData == null)
             return null;