"Liked Songs" isn't a real Spotify playlist — no playlist URI exists for it;
the web UI invents the virtual id 'spotify:liked-songs' and Spotify serves the
collection via the saved-tracks endpoint. The playlist DETAIL endpoint special-
cases that id, but the mirrored refresh path resolves stored ids through
get_playlist_by_id, which fed the virtual id straight into sp.playlist() ->
"http status: 400 ... Unsupported URL / URI" on every sync cycle, silently.
get_playlist_by_id now special-cases the virtual id at the client seam (every
by-id resolver benefits, not just the mirror adapter): it builds the Playlist
from the existing get_saved_tracks() pagination, with the real owner name and
track count. New LIKED_SONGS_PLAYLIST_ID constant owns the magic string.
Safety: get_saved_tracks swallows fetch errors into [] — indistinguishable
from "no likes" — and the virtual playlist is only ever offered when likes
exist. An empty result therefore resolves as a FAILED refresh (None) instead
of a valid-looking empty playlist a mirror sync might propagate by clearing
the server-side copy.
Tests: virtual id resolves from saved tracks and never touches the playlist
endpoint, real ids still do (regression), the mirrored adapter seam returns a
full PlaylistDetail, and empty saved-tracks -> None. 473 passed across the
playlist/mirror/spotify families.
