Users can now choose between iTunes/Apple Music and Deezer as their free
metadata source in Settings. Spotify always takes priority when authenticated;
the fallback handles all lookups when it's not.
Core changes:
- DeezerClient: full metadata interface (search, albums, artists, tracks)
matching iTunesClient's API surface with identical dataclass return types
- SpotifyClient: configurable _fallback property switches between iTunes/Deezer
based on live config reads (no restart needed)
- MetadataService, web_server, watchlist_scanner, api/search, repair_worker,
seasonal_discovery, personalized_playlists: all direct iTunesClient imports
replaced with fallback-aware helpers
Database:
- deezer_artist_id on watchlist_artists and similar_artists tables
- deezer_track_id/album_id/artist_id on discovery_pool and discovery_cache
- Full CRUD for Deezer IDs: add, read, update, backfill, metadata enrichment
- Watchlist duplicate detection by artist name prevents re-adding across sources
- SimilarArtist dataclass and all query/insert methods handle Deezer columns
Bug fixes found during review:
- Similar artist backfill was writing Deezer IDs into iTunes columns
- Discover hero was storing resolved Deezer IDs in wrong column
- Status cache not invalidating on settings save (source name lag)
- Watchlist add allowing duplicates when switching metadata sources
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