feat: add stable cover cache keys, Qobuz album-search fallback, metadata filters and extended sort options

- Introduce coverCacheKey parameter through Go backend and Kotlin bridge for stable SAF cover caching
- Add MetadataFromFilename flag to skip filename-only metadata and retry via temp-file copy
- Add Qobuz album-search fallback between API search and store scraping
- Extract buildReEnrichFFmpegMetadata to skip empty metadata fields
- Add metadata completeness filter (complete, missing year/genre/album artist)
- Add sort modes: artist, album, release date, genre (asc/desc)
- Prune stale library cover cache files after full scan
- Skip empty values and zero track/disc numbers in FFmpeg metadata
- Add new l10n keys for metadata filter and sort options

lib/services/ffmpeg_service.dart

@@ -1691,6 +1691,9 @@ class FFmpegService {
       final key = entry.key.toUpperCase();
       final normalizedKey = key.replaceAll(RegExp(r'[^A-Z0-9]'), '');
       final value = entry.value;
+      if (value.trim().isEmpty) {
+        continue;
+      }
 
       switch (normalizedKey) {
         case 'TITLE':
@@ -1708,12 +1711,16 @@ class FFmpegService {
         case 'TRACKNUMBER':
         case 'TRACK':
         case 'TRCK':
-          id3Map['track'] = value;
+          if (value != '0') {
+            id3Map['track'] = value;
+          }
           break;
         case 'DISCNUMBER':
         case 'DISC':
         case 'TPOS':
-          id3Map['disc'] = value;
+          if (value != '0') {
+            id3Map['disc'] = value;
+          }
           break;
         case 'DATE':
         case 'YEAR':