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>

ingest/radieo/subsonic.py

@@ -85,6 +85,17 @@ class SubsonicClient:
         body = self._get_json("getPlaylist", id=playlist_id)
         return body.get("playlist", {}).get("entry", [])
 
+    def search_songs(self, query: str, count: int = 25) -> list[dict]:
+        """Full-text song search (``search3``). Songs only, no artists/albums."""
+        body = self._get_json(
+            "search3",
+            query=query,
+            songCount=count,
+            artistCount=0,
+            albumCount=0,
+        )
+        return body.get("searchResult3", {}).get("song", [])
+
     def download(self, song_id: str, dest: Path, hint_ext: str | None = None) -> str:
         """Download a song to ``dest``; return the file extension used.