Milestone 5: MusicBrainz MBID canonicalizer

Give tracks a source-agnostic identity so the same song from different
sources no longer replays in a loop.

- Canonicalizer resolves (artist, title) to a MusicBrainz recording MBID
  (no API key; ~1 req/s, descriptive User-Agent, best-effort). Hits and
  confirmed misses are cached in SQLite; transient errors are not.
- Track.key becomes mbid:<id> when resolved, else a normalized
  name:<artist>|<title> fallback — still source-agnostic.
- Scheduler now owns the authoritative anti-repeat on the canonical key,
  canonicalizing the drawn track with a bounded retry; providers keep a
  cheap recent-locator filter to limit retries.
- db: canonical_cache table, history.locator column with migration for
  existing databases, recent_locators().
- Canonicalization can be turned off via RADIEO_CANONICAL_ENABLED=0.

Verified: MBID hit/cache/miss, cross-source key collapse, scheduler
dodging a recent play, schema migration, and full stack (Navidrome +
yt-dlp) with zero Python tracebacks and a valid 192 kbps MP3 stream.

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

docker-compose.yml

@@ -22,6 +22,9 @@ services:
       # Dosage du mix entre les sources (0 désactive).
       - RADIEO_WEIGHT_NAVIDROME=${RADIEO_WEIGHT_NAVIDROME:-3}
       - RADIEO_WEIGHT_YTDLP=${RADIEO_WEIGHT_YTDLP:-1}
+      # 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)}
     restart: unless-stopped
 
   stream: