Sixth cluster: the bulk-hydration get_<source>_playlist_states endpoints for
the five sources that build the identical per-entry dict + {"states": [...]}
shape (Tidal, Deezer, Qobuz, Spotify-Public, iTunes-Link) ->
core.discovery.endpoints.get_playlist_states(states, *, error_label,
info_log_label=None), wired via _get_source_playlist_states.
iTunes-Link is the only one of the five without the "Returning N stored ..."
info log, so info_log_label is optional (iTunes passes None to suppress it).
NOT folded in: the YouTube/ListenBrainz get_all_*_playlists endpoints. They
return {"playlists": [...]} (different key) with a different field set
(url / created_at / playlist, no discovery_results) and filter out
mirrored_/profile-scoped entries — genuinely divergent, kept as-is.
Tests: +4 (list build + last_accessed bump + exact shape, empty, optional ids
default None, missing-required-field -> 500). Full discovery suite: 183 passed.
web_server.py: -116 lines.
7b6615b65a