fix: install yt-dlp via pip3 for latest version, fix multi-artist folder path

- Replace apk yt-dlp with pip3 install to get current version with ytmsearch support
- Split mainArtist on comma in downloadTrack so multi-artist tracks
  always land in the primary artist folder (e.g. "Artist1, Artist2" → Artist1/)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -30,7 +30,8 @@ services:
       - MAX_PARALLEL=${MAX_PARALLEL:-2}
     command: >
       sh -c "
-        apk add --no-cache yt-dlp ffmpeg python3 &&
+        apk add --no-cache ffmpeg python3 py3-pip &&
+        pip3 install -U yt-dlp --break-system-packages &&
         npm install express cors &&
         node novasonic-server.js
       "