Milestone 4: yt-dlp provider and weighted source scheduler

Add a second playback source and a weighted scheduler mixing it with
Navidrome:

- Scheduler picks a provider by SOURCE_WEIGHTS, falling through to the
  others when one has nothing ready, so no source can stall playback.
- YtdlpProvider reads a hand-maintained config/urls.txt; container URLs
  (playlist/album/label/artist) are flat-extracted and one entry is
  drawn at random, honouring the anti-repeat window. Adds Track.source_url.
- YtdlpFetcher downloads bestaudio via the yt-dlp library, reusing the
  atomic hidden-temp-then-rename pattern; Liquidsoap decodes the result.
- Queue now dispatches to a fetcher registry keyed by backend.
- Sweep orphaned download temp files on daemon startup (leftovers from a
  killed container otherwise pile up and trip the stream fallback).

Verified end-to-end: yt-dlp opus decoded and served as 192 kbps MP3, and
the 3:1 default mix observed in play history.

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

docker-compose.yml

@@ -5,6 +5,7 @@ services:
     volumes:
       - ./cache:/cache          # volume partagé avec le stream (rw : téléchargements)
       - ./state:/state          # état persistant (SQLite) hors du cache éphémère
+      - ./config:/config:ro     # config éditable sans rebuild (urls.txt yt-dlp)
     environment:
       - RADIEO_CACHE_DIR=/cache
       - RADIEO_STATE_DIR=/state
@@ -16,6 +17,11 @@ services:
       - RADIEO_NAVIDROME_PASSWORD=${RADIEO_NAVIDROME_PASSWORD:-}
       - RADIEO_NAVIDROME_PLAYLIST=${RADIEO_NAVIDROME_PLAYLIST:-}
       - RADIEO_RETENTION_KEEP=${RADIEO_RETENTION_KEEP:-20}
+      # Source yt-dlp : liste d'URL dans config/urls.txt (créer depuis l'exemple).
+      - RADIEO_YTDLP_URLS_FILE=/config/urls.txt
+      # Dosage du mix entre les sources (0 désactive).
+      - RADIEO_WEIGHT_NAVIDROME=${RADIEO_WEIGHT_NAVIDROME:-3}
+      - RADIEO_WEIGHT_YTDLP=${RADIEO_WEIGHT_YTDLP:-1}
     restart: unless-stopped
 
   stream: