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_multiplatform.py

@@ -0,0 +1,79 @@
+import server.mf  # noqa: F401 — loads musicfetch, registers musicfetch_core
+import musicfetch_core as mf
+
+
+# ---- _sanitize_source ----
+def test_sanitize_source():
+    assert mf._sanitize_source("Youtube") == "youtube"
+    assert mf._sanitize_source("Soundcloud") == "soundcloud"
+    assert mf._sanitize_source("") == "downloads"
+
+
+# ---- _entry_to_hit ----
+def test_entry_to_hit_soundcloud_keeps_url_no_videoid():
+    h = mf._entry_to_hit({"id": "t1", "title": "Track", "uploader": "DJ",
+                          "ie_key": "Soundcloud", "url": "https://soundcloud.com/dj/track"})
+    assert h.payload["extractor"] == "soundcloud"
+    assert h.payload["url"] == "https://soundcloud.com/dj/track"
+    assert h.payload["videoId"] is None
+    assert h.artist == "DJ"
+
+
+def test_entry_to_hit_youtube_keeps_videoid():
+    h = mf._entry_to_hit({"id": "vid123", "title": "Song", "channel": "Chan",
+                          "ie_key": "Youtube", "url": "https://youtube.com/watch?v=vid123"})
+    assert h.payload["extractor"] == "youtube"
+    assert h.payload["videoId"] == "vid123"
+
+
+# ---- _track_url ----
+def test_track_url_youtube_prefers_music_youtube():
+    h = mf.Hit(source="youtube", kind="track", title="T", artist="A",
+               payload={"videoId": "vid", "extractor": "youtube", "url": "https://youtube.com/watch?v=vid"})
+    assert mf._track_url(h) == "https://music.youtube.com/watch?v=vid"
+
+
+def test_track_url_soundcloud_uses_native_url():
+    h = mf.Hit(source="youtube", kind="track", title="T", artist="A",
+               payload={"videoId": None, "extractor": "soundcloud", "url": "https://soundcloud.com/a/t"})
+    assert mf._track_url(h) == "https://soundcloud.com/a/t"
+
+
+def test_track_url_ytmusic_search_hit_default_youtube():
+    # ytmusicapi search hits carry only videoId (no extractor) -> music.youtube.
+    h = mf.Hit(source="youtube", kind="track", title="T", artist="A", payload={"videoId": "vid"})
+    assert mf._track_url(h) == "https://music.youtube.com/watch?v=vid"
+
+
+# ---- act_youtube routes to per-source folder ----
+def test_act_youtube_soundcloud_folder(monkeypatch):
+    captured = {}
+    monkeypatch.setattr(mf, "yt_download",
+                        lambda url, target, quality, dry_run, hit=None: captured.update(url=url, target=target) or True)
+    h = mf.Hit(source="youtube", kind="track", title="T", artist="DJ, Other",
+               payload={"videoId": None, "extractor": "soundcloud", "url": "https://soundcloud.com/dj/t"})
+    mf.act_youtube(h, "/media/music", "best", False)
+    assert captured["target"] == "/media/music/DJ/soundcloud"   # first artist only
+    assert captured["url"] == "https://soundcloud.com/dj/t"
+
+
+def test_act_youtube_youtube_folder(monkeypatch):
+    captured = {}
+    monkeypatch.setattr(mf, "yt_download",
+                        lambda url, target, quality, dry_run, hit=None: captured.update(target=target) or True)
+    h = mf.Hit(source="youtube", kind="track", title="T", artist="A",
+               payload={"videoId": "vid", "extractor": "youtube"})
+    mf.act_youtube(h, "/media/music", "best", False)
+    assert captured["target"] == "/media/music/A/youtube"
+
+
+# ---- download_single per-source folder ----
+def test_download_single_bandcamp_folder(monkeypatch):
+    monkeypatch.setattr(mf, "run_yt_dlp_get_metadata",
+                        lambda url: {"title": "Song", "artist": "Band", "extractor": "Bandcamp"})
+    captured = {}
+    monkeypatch.setattr(mf, "yt_download",
+                        lambda url, target, quality, dry_run, hit=None: captured.update(target=target) or True)
+    info = mf.download_single("https://band.bandcamp.com/track/song", "/media/music", "best", False)
+    assert captured["target"] == "/media/music/Band/bandcamp"
+    assert info == {"title": "Song", "artist": "Band", "ok": True}