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>

.gitignore

@@ -5,6 +5,9 @@
 # Secrets locaux
 .env
 
+# Liste de sources yt-dlp personnelle (garder seulement l'exemple)
+/config/urls.txt
+
 # État de l'ingestion (base SQLite persistante)
 /state/
 *.db