Add MusicBrainz watchlist artist ID storage, badges, linked-provider editing, and per-artist preferred source support.
Backfill watchlist MusicBrainz matches from already-enriched library artists so existing MusicBrainz worker matches appear in watchlist cards and settings.
Extend bulk watchlist add, liked artist matching, artist map source picking, and service status labels to recognize MusicBrainz, with regression tests for watchlist ID persistence and backfill.
