Generalize URL handling beyond YouTube to any yt-dlp-supported site
(SoundCloud, Bandcamp, etc), single tracks and playlists/sets/albums.
- probe_url(): one yt-dlp --flat-playlist probe classifies playlist vs track
and returns per-entry Hits; YouTube playlists still use ytmusicapi.
- _track_url(): YouTube tracks keep the music.youtube album-art URL; other
platforms download via their native entry URL (no more videoId reconstruction).
- Per-source folders: <root>/<artist>/<extractor>/ (soundcloud/bandcamp/youtube)
instead of hardcoded youtube; download_single derives source from metadata.
- download_hits() downloads pre-probed Hits; API probes once and passes hits
into the job closure. Replaces YouTube-only is_playlist_url/expand_playlist.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
