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");

DeepDrftTests/IndexSystemTests.cs

@@ -492,13 +492,13 @@ public class IndexSystemTests
             await database!.CreateVaultAsync(testVaultKey, MediaVaultType.Image);
 
             // Assert
-            Assert.That(database.HasIndexEntry(testVaultKey), Is.True, "Should contain added entry");
+            Assert.That(await database.HasIndexEntry(testVaultKey), Is.True, "Should contain added entry");
             Assert.That(database.GetIndexSize(), Is.EqualTo(1), "Should have one entry");
             Assert.That(File.Exists(IndexPath), Is.True, "Should persist to file");
 
             // Verify persistence by reloading database
             var reloadedDatabase = await FileDatabase.FromAsync(TestDirectory);
-            Assert.That(reloadedDatabase!.HasIndexEntry(testVaultKey), Is.True, "Should persist entry across restarts");
+            Assert.That(await reloadedDatabase!.HasIndexEntry(testVaultKey), Is.True, "Should persist entry across restarts");
         }
 
         [Test]
@@ -516,13 +516,13 @@ public class IndexSystemTests
             await vault!.AddEntryAsync(testKey, testImage);
 
             // Assert
-            Assert.That(vault.HasIndexEntry(testKey), Is.True, "Should contain added entry");
+            Assert.That(await vault.HasIndexEntry(testKey), Is.True, "Should contain added entry");
             Assert.That(vault.GetIndexSize(), Is.EqualTo(1), "Should have one entry");
             Assert.That(File.Exists(IndexPath), Is.True, "Should persist to file");
 
             // Verify persistence by reloading vault
             var reloadedVault = await ImageVault.FromAsync(TestDirectory);
-            Assert.That(reloadedVault!.HasIndexEntry(testKey), Is.True, "Should persist entry across restarts");
+            Assert.That(await reloadedVault!.HasIndexEntry(testKey), Is.True, "Should persist entry across restarts");
         }
     }
 }

DeepDrftTests/MediaVaultTests.cs

@@ -120,7 +120,7 @@ public class MediaVaultTests
             await _imageVault.AddEntryAsync(entryKey, imageBinary);
 
             // Assert
-            Assert.That(_imageVault.HasIndexEntry(entryKey), Is.True, "Should add to index");
+            Assert.That(await _imageVault.HasIndexEntry(entryKey), Is.True, "Should add to index");
             
             var expectedFilePath = Path.Combine(TestDirectory, "test-image.png");
             AssertMediaFileExists(expectedFilePath, imageBinary.Buffer);
@@ -148,7 +148,7 @@ public class MediaVaultTests
             
             foreach (var (key, binary) in entries)
             {
-                Assert.That(_imageVault.HasIndexEntry(key), Is.True, $"Should contain {key} in index");
+                Assert.That(await _imageVault.HasIndexEntry(key), Is.True, $"Should contain {key} in index");
                 var expectedFilePath = Path.Combine(TestDirectory, $"{key}.png");
                 AssertMediaFileExists(expectedFilePath, binary.Buffer);
             }
@@ -264,7 +264,7 @@ public class MediaVaultTests
             await _audioVault.AddEntryAsync(entryKey, audioBinary);
 
             // Assert
-            Assert.That(_audioVault.HasIndexEntry(entryKey), Is.True, "Should add to index");
+            Assert.That(await _audioVault.HasIndexEntry(entryKey), Is.True, "Should add to index");
             
             var expectedFilePath = Path.Combine(TestDirectory, "test-audio.mp3");
             AssertMediaFileExists(expectedFilePath, audioBinary.Buffer);
@@ -337,7 +337,7 @@ public class MediaVaultTests
                 return (string)method!.Invoke(_vault, new object[] { mediaKey })!;
             }
 
-            public bool HasIndexEntry(string entryId) => _vault.HasIndexEntry(entryId);
+            public Task<bool> HasIndexEntry(string entryId) => _vault.HasIndexEntry(entryId);
             public Task AddEntryAsync(string entryId, FileBinary media) => 
                 _vault.AddEntryAsync(entryId, media);
         }
@@ -423,7 +423,7 @@ public class MediaVaultTests
             await vault!.AddEntryAsync(entryKey, imageBinary);
 
             // Assert
-            Assert.That(vault.HasIndexEntry(entryKey), Is.True, "Should add entry to index");
+            Assert.That(await vault.HasIndexEntry(entryKey), Is.True, "Should add entry to index");
             
             var expectedFilePath = Path.Combine(TestDirectory, "test-media.png");
             AssertMediaFileExists(expectedFilePath, imageBinary.Buffer);