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>

.env.example

@@ -19,6 +19,13 @@ RADIEO_NAVIDROME_PLAYLIST=Radio
 RADIEO_WEIGHT_NAVIDROME=3
 RADIEO_WEIGHT_YTDLP=1
 
+# --- Canonicalizer MBID (optionnel) ---
+# Résout (artiste, titre) -> MBID MusicBrainz pour dédupliquer entre sources.
+# Aucune clé requise. Mettre 0 pour désactiver (clé = (artiste, titre)).
+RADIEO_CANONICAL_ENABLED=1
+# User-Agent envoyé à MusicBrainz (qui en exige un, descriptif).
+RADIEO_USER_AGENT=radieo/0.1 (personal music radio)
+
 # --- Rétention du cache (optionnel) ---
 # Nombre de morceaux joués conservés sur disque avant éviction (LRU).
 RADIEO_RETENTION_KEEP=20