Spotify album art: replace the 4-char size segment after '0000ab67616d'
with '82c1' to request the original uploaded master (up to 2000px+).
Applied via _upgrade_spotify_image_url() in Track, Artist, and Album
dataclass constructors and as a catch-all in _download_cover_art.
Scoped to the ab67616d album art prefix only — artist images use a
different prefix (ab676161) where the trick does not apply.
iTunes/Apple Music: replace '100x100bb' with '3000x3000bb' in all
artworkUrl100 replacements across Track, Artist, Album, and the
get_album images arrays. Also applied as a catch-all in _download_cover_art.
Deezer already uses cover_xl at its maximum — no changes needed there.