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>

tests/test_api_url.py

@@ -20,10 +20,17 @@ def _wait_done(client, auth, job_id, timeout=2.0):
     raise AssertionError("job never finished")
 
 
+def _mk_hit():
+    from server import mf
+    return mf.Hit(source="youtube", kind="track", title="t", artist="a", payload={"videoId": "1"})
+
+
 def test_playlist_url_batch_job(client, auth, monkeypatch):
-    monkeypatch.setattr("server.app.mf.download_playlist",
-                        lambda url, root, quality, dry_run: (2, 3, "My Mix"))
-    r = client.post("/fetch", params={"q": "https://music.youtube.com/playlist?list=PLx"}, headers=auth)
+    monkeypatch.setattr("server.app.mf.probe_url",
+                        lambda url: ("playlist", "My Mix", [_mk_hit(), _mk_hit(), _mk_hit()]))
+    monkeypatch.setattr("server.app.mf.download_hits",
+                        lambda hits, root, quality, dry_run: (2, 3))
+    r = client.post("/fetch", params={"q": "https://soundcloud.com/dj/sets/mix"}, headers=auth)
     assert r.status_code == 200
     body = r.json()
     assert body["status"] == "queued"
@@ -35,17 +42,20 @@ def test_playlist_url_batch_job(client, auth, monkeypatch):
 
 
 def test_playlist_zero_success_fails(client, auth, monkeypatch):
-    monkeypatch.setattr("server.app.mf.download_playlist",
-                        lambda url, root, quality, dry_run: (0, 3, "Dead Mix"))
+    monkeypatch.setattr("server.app.mf.probe_url",
+                        lambda url: ("playlist", "Dead Mix", [_mk_hit()]))
+    monkeypatch.setattr("server.app.mf.download_hits",
+                        lambda hits, root, quality, dry_run: (0, 3))
     body = client.post("/fetch", params={"q": "https://www.youtube.com/playlist?list=PLy"}, headers=auth).json()
     done = _wait_done(client, auth, body["job_id"])
     assert done["status"] == "failed"
 
 
 def test_single_video_url_download(client, auth, monkeypatch):
+    monkeypatch.setattr("server.app.mf.probe_url", lambda url: ("track", "Song", []))
     monkeypatch.setattr("server.app.mf.download_single",
                         lambda url, root, quality, dry_run: {"title": "Song", "artist": "A", "ok": True})
-    body = client.post("/fetch", params={"q": "https://music.youtube.com/watch?v=abc"}, headers=auth).json()
+    body = client.post("/fetch", params={"q": "https://soundcloud.com/a/song"}, headers=auth).json()
     assert body["hit"]["kind"] == "track"
     done = _wait_done(client, auth, body["job_id"])
     assert done["status"] == "done"