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>

.env.example

@@ -10,6 +10,15 @@ RADIEO_NAVIDROME_PASSWORD=monmotdepasse
 # Nom OU identifiant de la playlist à diffuser.
 RADIEO_NAVIDROME_PLAYLIST=Radio
 
+# --- Source yt-dlp ---
+# La liste d'URL se met dans config/urls.txt (copier config/urls.txt.example).
+# Rien à mettre ici ; le fichier absent désactive simplement la source.
+
+# --- Dosage du mix entre sources (optionnel) ---
+# Poids relatifs de tirage de chaque source (0 désactive la source).
+RADIEO_WEIGHT_NAVIDROME=3
+RADIEO_WEIGHT_YTDLP=1
+
 # --- Rétention du cache (optionnel) ---
 # Nombre de morceaux joués conservés sur disque avant éviction (LRU).
 RADIEO_RETENTION_KEEP=20