feat: multi-platform URL & playlist support via yt-dlp probe

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>

server/mf.py

@@ -25,10 +25,10 @@ act_lidarr_album = _mod.act_lidarr_album
 act_lidarr_artist = _mod.act_lidarr_artist
 QUALITY_CHOICES = _mod.QUALITY_CHOICES
 is_url = _mod.is_url
-is_playlist_url = _mod.is_playlist_url
-download_playlist = _mod.download_playlist
+probe_url = _mod.probe_url
+download_hits = _mod.download_hits
 download_single = _mod.download_single
 
 __all__ = ["Hit", "build_combined_hits", "pick", "act_youtube",
            "act_lidarr_album", "act_lidarr_artist", "QUALITY_CHOICES",
-           "is_url", "is_playlist_url", "download_playlist", "download_single"]
+           "is_url", "probe_url", "download_hits", "download_single"]