feat: always embed an album tag (default 'Unknown Album')

Downloads with no album (regular YouTube videos, fan edits, etc) left a blank
album, which trips up players like Plexamp. yt_download now appends
--parse-metadata "%(album|Unknown Album)s:%(meta_album)s" so the native/resolved
album is kept when present, else 'Unknown Album' is embedded. Applies to all
download paths (search, playlist, single URL).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_playlist.py

@@ -102,3 +102,11 @@ def test_yt_download_returns_true_on_zero_exit(monkeypatch):
 
 def test_yt_download_dry_run_returns_true():
     assert mf.yt_download("u", "/tmp/x", "best", True) is True
+
+
+def test_yt_download_always_sets_album_default(monkeypatch):
+    captured = {}
+    monkeypatch.setattr(mf.os, "makedirs", lambda *a, **k: None)
+    monkeypatch.setattr(mf.subprocess, "run", lambda cmd, **k: captured.update(cmd=cmd) or _CP(""))
+    mf.yt_download("u", "/tmp/x", "best", False)
+    assert "%(album|Unknown Album)s:%(meta_album)s" in captured["cmd"]