Lifts get_artist_map_data, get_artist_map_genre_list,
get_artist_map_genres, and get_artist_map_explore (plus the
_artmap_cache_* helpers and _artist_map_cache dict) to a new module.
Bodies are byte-identical to the originals. web_server.py keeps
thin route shells that delegate to the lifted functions.
A _SpotifyClientProxy resolves the global spotify_client lazily via
core.metadata.registry.get_spotify_client() so a Spotify re-auth that
rebinds the cached client stays visible to the lifted bodies.
web_server.py: 39124 → 38220 (-904 lines).
