perf: parallel I/O, caching, and chunked DB operations (batch 3)

- Orphan cleanup: parallel file existence checks (chunk 16)
- LocalLibraryState: O(1) findByTrackAndArtist via _byTrackKey map
- Local library load: parallel DB + SharedPreferences fetch
- Legacy mod-time backfill: chunked parallel File.stat (chunk 24)
- Downloaded album screen: cache disc groups, quality, cover path
- Local album screen: cache common quality, map-based batch delete
- Cache management: parallel async init, chunked directory cleanup
- Cover resolver: throttled preview exists check (2.2s interval)
- History/Library DB: chunked SQL DELETE (500 per batch)
- Batch delete screens: O(1) item lookup via tracksById map

lib/services/downloaded_embedded_cover_resolver.dart

@@ -21,6 +21,7 @@ class _EmbeddedCoverCacheEntry {
 class DownloadedEmbeddedCoverResolver {
   static const int _maxCacheEntries = 160;
   static const int _minModCheckIntervalMs = 1200;
+  static const int _minPreviewExistsCheckIntervalMs = 2200;
 
   static final LinkedHashMap<String, _EmbeddedCoverCacheEntry> _cache =
       LinkedHashMap<String, _EmbeddedCoverCacheEntry>();
@@ -28,6 +29,8 @@ class DownloadedEmbeddedCoverResolver {
   static final Set<String> _pendingModCheck = <String>{};
   static final Set<String> _failedExtract = <String>{};
   static final Map<String, int> _lastModCheckMillis = <String, int>{};
+  static final Map<String, int> _lastPreviewExistsCheckMillis =
+      <String, int>{};
 
   static String cleanFilePath(String? filePath) {
     if (filePath == null) return '';
@@ -64,12 +67,21 @@ class DownloadedEmbeddedCoverResolver {
 
     final cached = _cache[cleanPath];
     if (cached != null) {
-      if (File(cached.previewPath).existsSync()) {
+      final now = DateTime.now().millisecondsSinceEpoch;
+      final lastPreviewCheck = _lastPreviewExistsCheckMillis[cleanPath] ?? 0;
+      final shouldVerifyExists =
+          now - lastPreviewCheck >= _minPreviewExistsCheckIntervalMs;
+
+      if (!shouldVerifyExists || File(cached.previewPath).existsSync()) {
+        if (shouldVerifyExists) {
+          _lastPreviewExistsCheckMillis[cleanPath] = now;
+        }
         _touch(cleanPath, cached);
         _scheduleModCheck(cleanPath, onChanged: onChanged);
         return cached.previewPath;
       }
       _cache.remove(cleanPath);
+      _lastPreviewExistsCheckMillis.remove(cleanPath);
       _cleanupTempCoverPathSync(cached.previewPath);
     }
 
@@ -107,6 +119,7 @@ class DownloadedEmbeddedCoverResolver {
     _pendingModCheck.remove(cleanPath);
     _failedExtract.remove(cleanPath);
     _lastModCheckMillis.remove(cleanPath);
+    _lastPreviewExistsCheckMillis.remove(cleanPath);
     if (cached != null) {
       _cleanupTempCoverPathSync(cached.previewPath);
     }
@@ -129,6 +142,7 @@ class DownloadedEmbeddedCoverResolver {
       _pendingModCheck.remove(oldestKey);
       _failedExtract.remove(oldestKey);
       _lastModCheckMillis.remove(oldestKey);
+      _lastPreviewExistsCheckMillis.remove(oldestKey);
     }
   }
 
@@ -204,6 +218,8 @@ class DownloadedEmbeddedCoverResolver {
         );
         _touch(cleanPath, next);
         _failedExtract.remove(cleanPath);
+        _lastPreviewExistsCheckMillis[cleanPath] =
+            DateTime.now().millisecondsSinceEpoch;
         _trimCacheIfNeeded();
 
         if (previous != null && previous.previewPath != outputPath) {

lib/services/history_database.dart

@@ -525,12 +525,18 @@ class HistoryDatabase {
     if (ids.isEmpty) return 0;
 
     final db = await database;
-    final placeholders = List.filled(ids.length, '?').join(',');
-    final count = await db.rawDelete(
-      'DELETE FROM history WHERE id IN ($placeholders)',
-      ids,
-    );
-    _log.i('Deleted $count orphaned entries');
-    return count;
+    var totalDeleted = 0;
+    const chunkSize = 500;
+    for (var i = 0; i < ids.length; i += chunkSize) {
+      final end = (i + chunkSize < ids.length) ? i + chunkSize : ids.length;
+      final chunk = ids.sublist(i, end);
+      final placeholders = List.filled(chunk.length, '?').join(',');
+      totalDeleted += await db.rawDelete(
+        'DELETE FROM history WHERE id IN ($placeholders)',
+        chunk,
+      );
+    }
+    _log.i('Deleted $totalDeleted orphaned entries');
+    return totalDeleted;
   }
 }

lib/services/library_database.dart

@@ -229,6 +229,7 @@ class LibraryDatabase {
   }
   
   Future<void> upsertBatch(List<Map<String, dynamic>> items) async {
+    if (items.isEmpty) return;
     final db = await database;
     final batch = db.batch();
     
@@ -350,16 +351,46 @@ class LibraryDatabase {
   Future<int> cleanupMissingFiles() async {
     final db = await database;
     final rows = await db.query('library', columns: ['id', 'file_path']);
-    
-    int removed = 0;
-    for (final row in rows) {
-      final filePath = row['file_path'] as String;
-      if (!await fileExists(filePath)) {
-        await db.delete('library', where: 'id = ?', whereArgs: [row['id']]);
-        removed++;
+
+    final missingIds = <String>[];
+    const checkChunkSize = 16;
+    for (var i = 0; i < rows.length; i += checkChunkSize) {
+      final end = (i + checkChunkSize < rows.length)
+          ? i + checkChunkSize
+          : rows.length;
+      final chunk = rows.sublist(i, end);
+      final checks = await Future.wait<MapEntry<String, bool>>(
+        chunk.map((row) async {
+          final id = row['id'] as String;
+          final filePath = row['file_path'] as String;
+          return MapEntry(id, await fileExists(filePath));
+        }),
+      );
+      for (final check in checks) {
+        if (!check.value) {
+          missingIds.add(check.key);
+        }
       }
     }
-    
+
+    if (missingIds.isEmpty) {
+      return 0;
+    }
+
+    var removed = 0;
+    const deleteChunkSize = 500;
+    for (var i = 0; i < missingIds.length; i += deleteChunkSize) {
+      final end = (i + deleteChunkSize < missingIds.length)
+          ? i + deleteChunkSize
+          : missingIds.length;
+      final idChunk = missingIds.sublist(i, end);
+      final placeholders = List.filled(idChunk.length, '?').join(',');
+      removed += await db.rawDelete(
+        'DELETE FROM library WHERE id IN ($placeholders)',
+        idChunk,
+      );
+    }
+
     if (removed > 0) {
       _log.i('Cleaned up $removed missing files from library');
     }
@@ -440,14 +471,22 @@ class LibraryDatabase {
   Future<int> deleteByPaths(List<String> filePaths) async {
     if (filePaths.isEmpty) return 0;
     final db = await database;
-    final placeholders = List.filled(filePaths.length, '?').join(',');
-    final result = await db.rawDelete(
-      'DELETE FROM library WHERE file_path IN ($placeholders)',
-      filePaths,
-    );
-    if (result > 0) {
-      _log.i('Deleted $result items from library');
+    var totalDeleted = 0;
+    const chunkSize = 500;
+    for (var i = 0; i < filePaths.length; i += chunkSize) {
+      final end = (i + chunkSize < filePaths.length)
+          ? i + chunkSize
+          : filePaths.length;
+      final chunk = filePaths.sublist(i, end);
+      final placeholders = List.filled(chunk.length, '?').join(',');
+      totalDeleted += await db.rawDelete(
+        'DELETE FROM library WHERE file_path IN ($placeholders)',
+        chunk,
+      );
     }
-    return result;
+    if (totalDeleted > 0) {
+      _log.i('Deleted $totalDeleted items from library');
+    }
+    return totalDeleted;
   }
 }