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/app.py

@@ -53,14 +53,14 @@ def fetch(q: str = Query(..., min_length=1),
         raise HTTPException(status_code=422, detail=f"Invalid quality '{quality}'.")
 
     if mf.is_url(q):
-        kind = "playlist" if mf.is_playlist_url(q) else "track"
-        syn = mf.Hit(source="youtube", kind=kind, title="", artist="")
-        job = jobs.create_job(hit=syn, message=actions.url_started_message(kind))
+        kind, title, hits = mf.probe_url(q)
+        syn = mf.Hit(source="youtube", kind=kind, title=title, artist="")
+        job = jobs.create_job(hit=syn, message=actions.url_started_message(kind, title))
         response = _job_public(job)
         done_msg = actions.playlist_done_message if kind == "playlist" else actions.url_done_message
         jobs.run_job(
             job.id,
-            lambda: actions.perform_url_fetch(q, quality, ROOT),
+            lambda: actions.perform_url_fetch(q, kind, title, hits, quality, ROOT),
             done_message=done_msg,
             fail_message="Download failed.",
         )