ingest: support multiple weighted OpenSubsonic playlists

RADIEO_SUBSONIC_PLAYLIST now accepts a comma-separated list of playlist
names or ids, each optionally weighted with a '=<number>' suffix (e.g.
'Chill=3, Focus, Party=2'); default weight is 1 and 0 disables an entry.
The provider caches each playlist independently and walks them in
weighted-random order on every pick, falling through past empty, renamed
or unreachable playlists so the source never stalls. Picking the
playlist first (by weight) then a song uniformly gives each its
configured share regardless of length.

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

README.md

@@ -68,7 +68,9 @@ cp .env.example .env
 Fill in `.env`:
 
 - **OpenSubsonic server**: `RADIEO_SUBSONIC_URL` / `USER` / `PASSWORD` and the
-  playlist to broadcast in `RADIEO_SUBSONIC_PLAYLIST` (name or id). Works with
+  playlist(s) to broadcast in `RADIEO_SUBSONIC_PLAYLIST` — a comma-separated
+  list of names or ids, each optionally weighted with a `=<number>` suffix
+  (e.g. `Chill=3, Focus, Party=2`; default weight 1, `0` disables). Works with
   any OpenSubsonic-compatible server (Navidrome, Gonic, Airsonic…). Leave empty
   to disable this source. To make the player's "source" link work for library
   tracks, enable sharing on the server (Navidrome: `ND_ENABLESHARING=true`); the