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/actions.py

@@ -81,11 +81,11 @@ def url_done_message(result: dict) -> str:
     return f"Downloaded '{title}'." if title else "Download complete."
 
 
-def perform_url_fetch(url: str, quality: str, root: str) -> dict:
-    """Download a URL (playlist -> batch, else single). Raises if nothing
-    downloaded so the job is marked failed."""
-    if mf.is_playlist_url(url):
-        ok, total, title = mf.download_playlist(url, root, quality, False)
+def perform_url_fetch(url: str, kind: str, title: str, hits: list, quality: str, root: str) -> dict:
+    """Download a probed URL (playlist -> batch over pre-probed hits, else single).
+    Raises if nothing downloaded so the job is marked failed."""
+    if kind == "playlist":
+        ok, total = mf.download_hits(hits, root, quality, False)
         if ok == 0:
             raise RuntimeError(f"No tracks downloaded from playlist '{title}'." if title
                                else "No tracks downloaded from playlist.")