refactor(download): simplify finalization and remove the retired worker

lib/providers/download_queue_provider_finalization.dart

@@ -1150,7 +1150,10 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
         return null;
       }
       var preserve = false;
-      String? producedFileName;
+      final rawFileName =
+          (result['file_name'] as String?) ?? context.safFileName ?? 'track';
+      final newFileName =
+          '${rawFileName.replaceFirst(RegExp(r'\.[^.]+$'), '')}.flac';
       final newUri = await _replaceSafFileVia(
         uri: filePath,
         treeUri: treeUri,
@@ -1176,13 +1179,6 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
               'as FLAC and embedding metadata.',
             );
             await embedFlacMetadata(tempPath);
-            final rawFileName =
-                (result['file_name'] as String?) ??
-                context.safFileName ??
-                'track';
-            final baseName = rawFileName.replaceFirst(RegExp(r'\.[^.]+$'), '');
-            final newFileName = '$baseName.flac';
-            producedFileName = newFileName;
             return (tempPath, newFileName);
           }
           final flacPath = await FFmpegService.convertM4aToFlac(tempPath);
@@ -1191,13 +1187,6 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
           }
           addCleanup(flacPath);
           await embedFlacMetadata(flacPath);
-          final rawFileName =
-              (result['file_name'] as String?) ??
-              context.safFileName ??
-              'track';
-          final baseName = rawFileName.replaceFirst(RegExp(r'\.[^.]+$'), '');
-          final newFileName = '$baseName.flac';
-          producedFileName = newFileName;
           return (flacPath, newFileName);
         },
       );
@@ -1207,7 +1196,7 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
       if (newUri == null) {
         return null;
       }
-      result['file_name'] = producedFileName;
+      result['file_name'] = newFileName;
       markFinalOutputAsFlac();
       return newUri;
     }

lib/providers/download_queue_provider_single_item.dart

@@ -5,13 +5,6 @@ part of 'download_queue_provider.dart';
 /// the native download call, decrypt/convert/embed finalization, and the
 /// history hand-off. Queue scheduling stays in the main file.
 extension _SingleItemDownload on DownloadQueueNotifier {
-  bool _isStorageWriteFailure(Map<String, dynamic> result) {
-    return isStorageWriteFailure(
-      errorType: result['error_type']?.toString(),
-      errorMessage: (result['error'] ?? result['message'])?.toString(),
-    );
-  }
-
   Future<String?> _runPostProcessingHooks(
     String filePath,
     Track track,
@@ -520,7 +513,11 @@ class _DownloadRun {
       outputDir: effectiveOutputDir,
     );
 
-    if (result['success'] != true && n._isStorageWriteFailure(result)) {
+    if (result['success'] != true &&
+        isStorageWriteFailure(
+          errorType: result['error_type']?.toString(),
+          errorMessage: (result['error'] ?? result['message'])?.toString(),
+        )) {
       if (n._isLocallyCancelled(item.id)) {
         _log.i('Download was cancelled before storage fallback, skipping');
         return false;
@@ -931,7 +928,7 @@ class _DownloadRun {
 
     if (shouldForceDashSafM4aHandling) {
       _log.w(
-        'SAF file is labeled FLAC but backend returned DASH/M4A stream; converting it back to FLAC.',
+        'SAF file is labeled FLAC but backend returned an M4A stream; converting it back to FLAC.',
       );
     }
 
@@ -1268,10 +1265,8 @@ class _DownloadRun {
               : renamedPath;
           await file.rename(finalRenamedPath);
           targetPath = finalRenamedPath;
-          filePath = finalRenamedPath;
-        } else {
-          filePath = targetPath;
         }
+        filePath = targetPath;
 
         if (metadataEmbeddingEnabled) {
           n.updateItemStatus(
@@ -1288,9 +1283,7 @@ class _DownloadRun {
   }
 
   Future<void> _convertLocalM4aToFlac(String currentFilePath) async {
-    _log.d(
-      'M4A file detected (Hi-Res DASH stream), attempting conversion to FLAC...',
-    );
+    _log.d('M4A file detected, attempting conversion to FLAC...');
 
     try {
       final file = File(currentFilePath);
@@ -1348,23 +1341,7 @@ class _DownloadRun {
               _log.d('Converted to FLAC: $flacPath');
 
               _log.d('Embedding metadata and cover to converted FLAC...');
-              try {
-                final backendGenre = result['genre'] as String?;
-                final backendLabel = result['label'] as String?;
-                final backendCopyright = result['copyright'] as String?;
-                if (backendGenre != null ||
-                    backendLabel != null ||
-                    backendCopyright != null) {
-                  _log.d(
-                    'Extended metadata from backend - Genre: $backendGenre, Label: $backendLabel, Copyright: $backendCopyright',
-                  );
-                }
-
-                await _embedFinalMetadata(flacPath, format: 'flac');
-                _log.d('Metadata and cover embedded successfully');
-              } catch (e) {
-                _log.w('Warning: Failed to embed metadata/cover: $e');
-              }
+              await _embedFinalMetadata(flacPath, format: 'flac');
             } else {
               _log.w('FFmpeg conversion returned null, keeping M4A file');
             }
@@ -1384,16 +1361,17 @@ class _DownloadRun {
         resultOutputExt == '.ogg';
     final isMp3File =
         currentFilePath.endsWith('.mp3') || resultOutputExt == '.mp3';
-    final ext = isOpusFile
-        ? (resultOutputExt == '.ogg' ? '.ogg' : '.opus')
+    final (ext, formatName) = isOpusFile
+        ? (resultOutputExt == '.ogg' ? '.ogg' : '.opus', 'Opus')
         : isMp3File
-        ? '.mp3'
-        : '.flac';
-    final formatName = isOpusFile
-        ? 'Opus'
-        : isMp3File
-        ? 'MP3'
-        : 'FLAC';
+        ? ('.mp3', 'MP3')
+        : ('.flac', 'FLAC');
+    // MP3 takes precedence for the embed format when both checks match.
+    final embedFormat = isMp3File
+        ? 'mp3'
+        : isOpusFile
+        ? 'opus'
+        : 'flac';
     _log.d(
       'SAF $formatName detected, embedding metadata and cover via temp file...',
     );
@@ -1416,11 +1394,6 @@ class _DownloadRun {
           // so a sidecar .lrc written next to it would be orphaned;
           // the SAF .lrc is written by _saveExternalLrc after publish,
           // reusing the LRC fetched here via result['lyrics_lrc'].
-          final embedFormat = isMp3File
-              ? 'mp3'
-              : isOpusFile
-              ? 'opus'
-              : 'flac';
           final fetchedLrc = await _embedFinalMetadata(
             tempPath,
             format: embedFormat,

lib/services/batch_metadata_re_enrich.dart

@@ -1,7 +1,7 @@
 import 'package:spotiflac_android/models/settings.dart';
 import 'package:spotiflac_android/services/library_database.dart';
 
-/// Field group keys understood by the Go re-enrich backend.
+/// Field group keys understood by the native re-enrich backend.
 class ReEnrichFields {
   static const String cover = 'cover';
   static const String lyrics = 'lyrics';

lib/services/ffmpeg_service.dart

@@ -505,16 +505,6 @@ class FFmpegService {
     }.contains(normalized);
   }
 
-  /// Probes the source audio bit depth (bits_per_raw_sample, falling back to
-  /// bits_per_sample). Returns null when unknown.
-  static Future<int?> probeBitDepth(String filePath) async {
-    return (await _probePrimaryAudioProperties(filePath)).bitDepth;
-  }
-
-  static Future<int?> probeSampleRate(String filePath) async {
-    return (await _probePrimaryAudioProperties(filePath)).sampleRate;
-  }
-
   /// Returns `true` when [filePath] starts with the native FLAC magic bytes
   /// (`fLaC`). Useful to distinguish a real FLAC file from a FLAC-in-MP4
   /// container that carries a `.flac` extension or claims codec=flac.
@@ -2102,8 +2092,9 @@ class FFmpegService {
   }
 
   /// Convert to uncompressed PCM (WAV or AIFF), preserving bit depth when known.
-  /// Tags and cover are written natively into an embedded ID3 chunk by the Go
-  /// backend (RIFF "id3 " for WAV, "ID3 " for AIFF) for full-fidelity tagging.
+  /// Tags and cover are written natively into an embedded ID3 chunk by the
+  /// native backend (RIFF "id3 " for WAV, "ID3 " for AIFF) for full-fidelity
+  /// tagging.
   static Future<String?> _convertToPcm({
     required String inputPath,
     required Map<String, String> metadata,
@@ -2125,7 +2116,7 @@ class FFmpegService {
     final outputPath = outputPlan.workingPath;
     var depth = targetBitDepth ?? sourceBitDepth;
     if (depth == null || depth <= 0) {
-      depth = await probeBitDepth(inputPath);
+      depth = (await _probePrimaryAudioProperties(inputPath)).bitDepth;
     }
     final use24 = depth != null && depth >= 24;
     final codec = isAiff
@@ -2198,9 +2189,9 @@ class FFmpegService {
     );
   }
 
-  /// Writes tags + cover into a WAV/AIFF file via the Go native ID3-chunk
+  /// Writes tags + cover into a WAV/AIFF file via the native backend ID3-chunk
   /// writer (PlatformBridge.editFileMetadata). Maps Vorbis-style metadata keys
-  /// to the lowercase field names the Go editor expects.
+  /// to the lowercase field names the native editor expects.
   static Future<bool> _embedChunkTagsNative(
     String path,
     Map<String, String> vorbisMetadata,
@@ -2284,7 +2275,7 @@ class FFmpegService {
   /// Each track is extracted with `-c copy` (no re-encoding) and metadata is embedded.
   /// [audioPath] is the source audio file (FLAC, WAV, etc.)
   /// [outputDir] is where individual track files will be saved
-  /// [tracks] is the list of track split info from the Go CUE parser
+  /// [tracks] is the list of track split info from the native CUE parser
   /// [albumMetadata] contains album-level metadata (artist, album, genre, date)
   static Future<List<String>?> splitCueToTracks({
     required String audioPath,