SimilarArtists() was calling api.listenbrainz.org/1/explore/similar-artists/{mbid}
which doesn't exist (404). Changed to labs.api.listenbrainz.org/similar-artists/json
with artist_mbids + algorithm query params — the same Labs API that searchindex.go
already uses for Tier 4 indexing.
Also added snake_case → camelCase wire conversion via existing lbSimilarArtistWire type.