Adds an opt-in no-creds Spotify metadata path: SpotifyClient serves SpotipyFree
data (real Spotify IDs) when metadata.spotify_free is enabled AND SpotipyFree is
installed AND there's no real Spotify auth. The data lands in the SAME spotify_*
columns, so the enrichment worker, search, and #775 lookups work UNCHANGED —
they just receive free-sourced data. The worker's only change is its availability
gate.
- core/spotify_free_metadata.py: SpotifyFreeMetadataClient + normalize_artist +
pure gate fns (should_use_free_fallback / should_offer_spotify_metadata /
spotify_free_installed).
- SpotifyClient: _free_enabled (opt-in, default OFF) / _free_available /
is_spotify_metadata_available / _free_active + in-client routing for
search_artists/tracks + get_album/artist/track_details/album_tracks/artist_albums.
- 3 scoped availability gates use is_spotify_metadata_available(): enrichment
worker loop, search resolve_client, watchlist. The ~40 discovery/user-library
sites stay auth-only (no sprawl, no user-data risk).
Authed users are untouched (gate closed when auth healthy). UI surfacing comes
next. Pure gates + normalizer tested; orchestrator resolve test added.
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