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>

.env.example

@@ -14,10 +14,18 @@ RADIEO_NAVIDROME_PLAYLIST=Radio
 # La liste d'URL se met dans config/urls.txt (copier config/urls.txt.example).
 # Rien à mettre ici ; le fichier absent désactive simplement la source.
 
+# --- Source ListenBrainz ---
+# Feed Atom de recommandations. URL http(s) du feed de syndication, ou chemin
+# local sous /config (ex. /config/recommendations.xml) pour tester. Vide = off.
+# ListenBrainz ne fait que *nommer* des morceaux : chacun est résolu vers
+# Navidrome puis, à défaut, yt-dlp.
+RADIEO_LISTENBRAINZ_URL=https://listenbrainz.org/syndication-feed/user/monuser/recommendations/weekly-exploration
+
 # --- Dosage du mix entre sources (optionnel) ---
 # Poids relatifs de tirage de chaque source (0 désactive la source).
 RADIEO_WEIGHT_NAVIDROME=3
 RADIEO_WEIGHT_YTDLP=1
+RADIEO_WEIGHT_LISTENBRAINZ=2
 
 # --- Canonicalizer MBID (optionnel) ---
 # Résout (artiste, titre) -> MBID MusicBrainz pour dédupliquer entre sources.