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>

ingest/Dockerfile

@@ -2,8 +2,9 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-# Milestone 2 uses the standard library only; third-party dependencies
-# (httpx, yt-dlp, feedparser…) will be added in later milestones.
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+
 COPY radieo/ ./radieo/
 
 ENV PYTHONUNBUFFERED=1