Milestone 6: ListenBrainz recommendations provider

Add a third playback source: a ListenBrainz recommendations Atom feed. Each
suggestion already carries a MusicBrainz recording MBID, title and artist, so
it is keyed directly by MBID (source-agnostic identity, no extra lookup) and
resolved to a concrete file — Navidrome search3 first, then a yt-dlp
ytsearch1: fallback.

- providers/listenbrainz.py: parse the Atom/HTML feed, anti-repeat on the MBID
  key, resolve Navidrome-then-yt-dlp. Feed may be an http(s) URL or a local
  path (for testing).
- subsonic.py: add search_songs (search3) for resolution.
- canonicalizer.py: short-circuit when a Track already has an MBID, so
  feed-provided MBIDs are trusted and MusicBrainz is not hit.
- __main__.py: wire the provider in; register the yt-dlp fetcher as a
  resolution backend even when the yt-dlp source is off; close providers on
  shutdown.
- config/compose/.env.example: RADIEO_LISTENBRAINZ_URL + weight.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -19,9 +19,13 @@ services:
       - RADIEO_RETENTION_KEEP=${RADIEO_RETENTION_KEEP:-20}
       # Source yt-dlp : liste d'URL dans config/urls.txt (créer depuis l'exemple).
       - RADIEO_YTDLP_URLS_FILE=/config/urls.txt
+      # Source ListenBrainz : URL du feed de recommandations (ou chemin local
+      # sous /config, ex. /config/recommendations.xml). Vide désactive la source.
+      - RADIEO_LISTENBRAINZ_URL=${RADIEO_LISTENBRAINZ_URL:-}
       # Dosage du mix entre les sources (0 désactive).
       - RADIEO_WEIGHT_NAVIDROME=${RADIEO_WEIGHT_NAVIDROME:-3}
       - RADIEO_WEIGHT_YTDLP=${RADIEO_WEIGHT_YTDLP:-1}
+      - RADIEO_WEIGHT_LISTENBRAINZ=${RADIEO_WEIGHT_LISTENBRAINZ:-2}
       # Canonicalizer MusicBrainz (identité MBID inter-sources ; sans clé).
       - RADIEO_CANONICAL_ENABLED=${RADIEO_CANONICAL_ENABLED:-1}
       - RADIEO_USER_AGENT=${RADIEO_USER_AGENT:-radieo/0.1 (personal music radio)}