feat(worker): Last.fm artist.getSimilar as a second discovery SimilaritySource

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.

worker/lyra_worker/registry.py

@@ -12,6 +12,7 @@ from lyra_worker.adapters.youtube import YouTubeAdapter
 from lyra_worker.browser import MbBrowser
 from lyra_worker.probe import AudioProbe
 from lyra_worker.resolver import MbResolver
+from lyra_worker.similarity._lastfm import LastfmSource
 from lyra_worker.similarity._listenbrainz import ListenBrainzSource
 from lyra_worker.similarity.base import SimilaritySource
 from lyra_worker.tagger import Tagger
@@ -53,5 +54,12 @@ def build_probe() -> AudioProbe:
 
 def build_similarity_sources(config: dict) -> list[SimilaritySource]:
     """Similarity sources for discovery. ListenBrainz needs no credentials, so it is
-    always constructed; per-run reachability is checked via each source's health()."""
-    return [ListenBrainzSource(base_url=config.get("discover.listenBrainzUrl") or "")]
+    always constructed; per-run reachability is checked via each source's health().
+    Last.fm is added only when an API key is configured."""
+    sources: list[SimilaritySource] = [
+        ListenBrainzSource(base_url=config.get("discover.listenBrainzUrl") or "")
+    ]
+    key = config.get("lastfm.api_key")
+    if key:
+        sources.append(LastfmSource(api_key=key, browser=MusicBrainzBrowser()))
+    return sources