Lifts _match_liked_artists_to_all_sources and
_backfill_liked_artist_images. Both bodies are byte-identical to the
originals. Uses the same _SpotifyClientProxy + _get_*_client shim
pattern as core/artists/map.py so the bodies resolve their original
names without modification.
web_server.py: 37501 → 37245 (-256 lines).
