feat: implement MusicBrainz genre enrichment for all external sources

- Add automatic genre enrichment for Deezer, Qobuz, and SquidWTF tracks
- Use ISRC codes for exact matching, fallback to title/artist search
- Cache results in Redis (30 days) and file cache for performance
- Remove unused Spotify API ClientId and ClientSecret settings
- Respect MusicBrainz rate limits (1 req/sec)

This ensures all external tracks have genre metadata, even when the
source provider doesn't include it (especially SquidWTF/Tidal).

docker-compose.yml

@@ -107,8 +107,6 @@ services:
       
       # ===== SPOTIFY DIRECT API (for lyrics, ISRC matching, track ordering) =====
       - SpotifyApi__Enabled=${SPOTIFY_API_ENABLED:-false}
-      - SpotifyApi__ClientId=${SPOTIFY_API_CLIENT_ID:-}
-      - SpotifyApi__ClientSecret=${SPOTIFY_API_CLIENT_SECRET:-}
       - SpotifyApi__SessionCookie=${SPOTIFY_API_SESSION_COOKIE:-}
       - SpotifyApi__SessionCookieSetDate=${SPOTIFY_API_SESSION_COOKIE_SET_DATE:-}
       - SpotifyApi__CacheDurationMinutes=${SPOTIFY_API_CACHE_DURATION_MINUTES:-60}