Milestone 3: Navidrome (OpenSubsonic) playback provider

Replace the directory-scan queue with a real ingestion pipeline:
provider -> fetcher -> cache -> ready queue, driven by a background
prefetch thread.

- subsonic.py: minimal OpenSubsonic client (salted-token auth,
  getPlaylists/getPlaylist, raw streaming download).
- providers/navidrome.py: pick tracks from a playlist (by name or id),
  with anti-repeat and periodic playlist reload.
- fetchers/subsonic.py: atomic download into the shared cache.
- db.py: SQLite state — append-only play history (anti-repeat + stats)
  and cache_files LRU retention (keep the N most recently played).
- queue.py: prefetch buffer + retention on play; graceful degradation
  to the stream's local-cache fallback when no source is configured.
- api.py: GET /next now carries real title/artist metadata.
- Config via .env (Navidrome credentials), persistent state/ volume,
  httpx dependency.

Verified end-to-end against a live Navidrome: playlist resolved,
tracks downloaded and broadcast, retention and history correct.

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

.env.example

@@ -0,0 +1,15 @@
+# radieo — configuration locale. Copier en `.env` et remplir.
+# docker compose lit automatiquement `.env` pour ces variables.
+
+# --- Source Navidrome / OpenSubsonic ---
+# URL de base de ton serveur (sans /rest). Laisser les champs vides désactive
+# la source : le stream joue alors uniquement les fichiers déjà dans cache/.
+RADIEO_NAVIDROME_URL=https://navidrome.example.org
+RADIEO_NAVIDROME_USER=monuser
+RADIEO_NAVIDROME_PASSWORD=monmotdepasse
+# Nom OU identifiant de la playlist à diffuser.
+RADIEO_NAVIDROME_PLAYLIST=Radio
+
+# --- Rétention du cache (optionnel) ---
+# Nombre de morceaux joués conservés sur disque avant éviction (LRU).
+RADIEO_RETENTION_KEEP=20