Adds LastfmSource, resolving Last.fm's often-empty artist mbid via the
injected MB browser (name->mbid cached) so every emitted SimilarArtist.mbid
is a real MusicBrainz artist MBID. Registered in build_similarity_sources,
gated on lastfm.api_key so the ListenBrainz-only path is unchanged when
unset.
The placeholder algorithm string (contribution_5_threshold_15_limit_50) is not a
permitted enum member of the LB Labs endpoint — every lookup 400'd, making discovery
inert. Verified against the live API 2026-07-12: the days_7500/contribution_3/
threshold_10/limit_100 variant returns rows shaped {artist_mbid,name,comment,score},
which the existing defensive parser already handles. Fixed in both the worker adapter
and the web seed-search client (kept in sync).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Jonathan