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>

docker-compose.yml

@@ -3,10 +3,19 @@ services:
     build: ./ingest
     image: radieo-ingest
     volumes:
-      - ./cache:/cache          # volume partagé avec le stream
+      - ./cache:/cache          # volume partagé avec le stream (rw : téléchargements)
+      - ./state:/state          # état persistant (SQLite) hors du cache éphémère
     environment:
       - RADIEO_CACHE_DIR=/cache
+      - RADIEO_STATE_DIR=/state
       - RADIEO_HTTP_PORT=8080
+      # Source Navidrome / OpenSubsonic (voir .env / .env.example).
+      # Laisser vide désactive la source : le stream joue alors son cache local.
+      - RADIEO_NAVIDROME_URL=${RADIEO_NAVIDROME_URL:-}
+      - RADIEO_NAVIDROME_USER=${RADIEO_NAVIDROME_USER:-}
+      - RADIEO_NAVIDROME_PASSWORD=${RADIEO_NAVIDROME_PASSWORD:-}
+      - RADIEO_NAVIDROME_PLAYLIST=${RADIEO_NAVIDROME_PLAYLIST:-}
+      - RADIEO_RETENTION_KEEP=${RADIEO_RETENTION_KEEP:-20}
     restart: unless-stopped
 
   stream: