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>

config/urls.txt.example

@@ -0,0 +1,14 @@
+# radieo — liste de sources yt-dlp. Copier en `config/urls.txt`.
+# Une URL par ligne. Les lignes vides et celles commençant par '#' sont ignorées.
+#
+# Chaque URL peut être :
+#   - un morceau précis            (téléchargé tel quel)
+#   - une playlist / album / label / page d'artiste
+#     -> radieo y pioche un morceau au hasard à chaque tour,
+#        en évitant ceux joués récemment.
+#
+# Exemples (à remplacer par les tiens) :
+# https://www.youtube.com/watch?v=dQw4w9WgXcQ
+# https://soundcloud.com/artiste/un-morceau
+# https://artiste.bandcamp.com/album/un-album
+# https://www.youtube.com/playlist?list=PLxxxxxxxx