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

DeepDrftTests/FileDatabaseTests.cs

@@ -92,7 +92,7 @@ public class FileDatabaseTests
         // Assert
         var vault = _fileDatabase.GetVault(TestData.TestKeys.ImageVaultKey);
         Assert.That(vault, Is.Not.Null, "Vault should not be null");
-        Assert.That(vault!.HasIndexEntry(TestData.TestKeys.TestImageEntry), Is.True, 
+        Assert.That(await vault!.HasIndexEntry(TestData.TestKeys.TestImageEntry), Is.True,
             "Added image should be in the index");
     }
 
@@ -182,7 +182,7 @@ public class FileDatabaseTests
         Assert.That(reloadedDatabase.GetIndexSize(), Is.EqualTo(1), "Index count should be 1");
         
         // Verify vault exists
-        Assert.That(reloadedDatabase.HasIndexEntry(TestData.TestKeys.ImageVaultKey), Is.True, 
+        Assert.That(await reloadedDatabase.HasIndexEntry(TestData.TestKeys.ImageVaultKey), Is.True,
             "Vault should be present in index");
         Assert.That(reloadedDatabase.HasVault(TestData.TestKeys.ImageVaultKey), Is.True, 
             "Vault should be present in vault collection");