Group YouTube playlist downloads into a single album bucket via playlist metadata fallback

[Copilot]

YouTube playlist imports were often landing as a flat list of songs because many tracks lacked standard album tags. This change makes playlist metadata act as the album source when album tags are missing, so playlist tracks group as an album in the library view.

• Album resolution logic

  • Keep existing precedence for real album tags (TALB, album, ©alb).
  • Add fallback support for playlist metadata tags (TXXX:PLAYLIST, TXXX:playlist, playlist).
  • Refactor tag-value extraction into a small reusable helper (Library._tag_text) for clearer parsing behavior.

• Behavioral guarantees

  • Playlist metadata is used only as fallback, not as override.
  • If both album and playlist metadata exist, album metadata still wins.

• Focused coverage

  • Added tests for:
    • playlist fallback when album metadata is absent
    • album-tag precedence when both album and playlist metadata are present

for key in ("album", "©alb"):
    if key in tags:
        text = self._tag_text(tags[key])
        if text:
            return text

for key in ("TXXX:PLAYLIST", "TXXX:playlist", "playlist"):
    if key in tags:
        text = self._tag_text(tags[key])
        if text:
            return text