feat: propagate download cancellation through entire pipeline, add MusicBrainz album artist fallback, and allow disabling home feed

- Add reference-counted cancel entries to prevent premature cleanup when multiple operations share the same itemID
- Propagate cancellation to DownloadTrack, DownloadWithFallback, DownloadWithExtensionsJSON, extension providers, and ISRC search
- Fetch album artist from MusicBrainz when missing during download and re-enrich
- Make ALBUMARTIST tag nullable to avoid writing artistName as album artist
- Add home feed 'Off' option in extension settings
- Skip deezer in download provider priority sanitization

lib/models/settings.dart

@@ -5,6 +5,8 @@ part 'settings.g.dart';
 
 @JsonSerializable()
 class AppSettings {
+  static const String homeFeedProviderOff = '__off__';
+
   final String defaultService;
   final String audioQuality;
   final String filenameFormat;