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.
Adds registry.build_similarity_sources() (always constructs
ListenBrainzSource; reachability is a per-run health() concern, not a
startup gate) and wires discovery into the worker's main loop: a
scheduled sweep gated on discover.enabled + DISCOVER_TICK_SECONDS, and
a one-shot discover.requested trigger mirroring the existing
scan.requested pattern.
Also hardens run_discovery's health-check pass: a source whose
health() raises is now logged and skipped instead of aborting the
whole sweep (accepted finding from the Task-3 review).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Jonathan