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/tests/test_similarity_registry.py

@@ -1,4 +1,5 @@
 from lyra_worker.registry import build_similarity_sources
+from lyra_worker.similarity._lastfm import LastfmSource
 from lyra_worker.similarity._listenbrainz import ListenBrainzSource
 
 
@@ -12,3 +13,17 @@ def test_build_returns_listenbrainz_by_default():
 def test_base_url_override_from_config():
     src = build_similarity_sources({"discover.listenBrainzUrl": "http://lb.local"})[0]
     assert src._base == "http://lb.local"
+
+
+def test_lastfm_added_when_api_key_configured():
+    sources = build_similarity_sources({"lastfm.api_key": "secret-key"})
+    names = [s.name for s in sources]
+    assert names == ["listenbrainz", "lastfm"]
+    lastfm = next(s for s in sources if s.name == "lastfm")
+    assert isinstance(lastfm, LastfmSource)
+    assert lastfm._key == "secret-key"
+
+
+def test_lastfm_omitted_without_api_key():
+    sources = build_similarity_sources({})
+    assert all(s.name != "lastfm" for s in sources)