A 4xx/5xx (rate limit / auth) now raises instead of parsing an error body;
callers already handle it (discovery per-source try/except; album fetch
returns []). Closes the last small deferred cleanup.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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.
Jonathan