The cover picker only offered CAA covers from a song's MATCHED release, so an
unmatched song (the city-pop pile) got nothing but Current/Pack/Upload/URL. Add
a search box: GET /api/song/{fn}/art/cover-search?q= searches MusicBrainz
release-groups and returns each album's CAA front-250 thumb; the picker renders
them as pickable tiles (same apply→/art/url path; covers with no CAA art
self-hide). Pre-filled from the song's artist + album/title (romaji fallback), so
a blank-artist pack pre-fills "Junko Yagami …". Reuses the throttled _mb_http_get.
