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.
Lifts _match_liked_artists_to_all_sources and
_backfill_liked_artist_images. Both bodies are byte-identical to the
originals. Uses the same _SpotifyClientProxy + _get_*_client shim
pattern as core/artists/map.py so the bodies resolve their original
names without modification.
web_server.py: 37501 → 37245 (-256 lines).