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_lastfm_source.py

@@ -0,0 +1,108 @@
+from unittest.mock import MagicMock, patch
+
+import requests
+
+from lyra_worker.browser import ArtistHit
+from lyra_worker.similarity._lastfm import LastfmSource
+from lyra_worker.similarity.base import SimilarArtist
+
+
+def _resp(payload, status=200):
+    r = MagicMock()
+    r.status_code = status
+    r.json.return_value = payload
+    return r
+
+
+class _FakeBrowser:
+    """Records search_artist calls; resolves via a name -> [ArtistHit] map."""
+
+    def __init__(self, hits=None):
+        self._hits = dict(hits or {})
+        self.calls = []
+
+    def search_artist(self, name):
+        self.calls.append(name)
+        return list(self._hits.get(name, []))
+
+
+def test_similar_artists_parses_and_uses_supplied_mbid():
+    payload = {"similarartists": {"artist": [
+        {"name": "Cand One", "mbid": "c1", "match": "0.9"},
+    ]}}
+    browser = _FakeBrowser()
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        out = LastfmSource(api_key="k", browser=browser).similar_artists("seed")
+    assert out == [SimilarArtist("c1", "Cand One", 0.9)]
+    assert browser.calls == []  # mbid supplied, no resolution needed
+
+
+def test_similar_artists_resolves_missing_mbid_and_caches():
+    payload = {"similarartists": {"artist": [
+        {"name": "Cand Two", "mbid": "", "match": "0.5"},
+        {"name": "Cand Two", "mbid": "", "match": "0.4"},
+    ]}}
+    browser = _FakeBrowser(hits={"Cand Two": [ArtistHit(mbid="c2", name="Cand Two")]})
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        out = LastfmSource(api_key="k", browser=browser).similar_artists("seed")
+    assert out == [
+        SimilarArtist("c2", "Cand Two", 0.5),
+        SimilarArtist("c2", "Cand Two", 0.4),
+    ]
+    assert browser.calls == ["Cand Two"]  # cached after first lookup
+
+
+def test_similar_artists_skips_unresolvable_row():
+    payload = {"similarartists": {"artist": [
+        {"name": "Unknown Artist", "mbid": "", "match": "0.5"},
+        {"name": "Cand One", "mbid": "c1", "match": "0.9"},
+    ]}}
+    browser = _FakeBrowser(hits={})  # no hits for "Unknown Artist"
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        out = LastfmSource(api_key="k", browser=browser).similar_artists("seed")
+    assert out == [SimilarArtist("c1", "Cand One", 0.9)]
+
+
+def test_similar_artists_skips_self():
+    payload = {"similarartists": {"artist": [
+        {"name": "Seed", "mbid": "seed", "match": "1.0"},
+        {"name": "Cand One", "mbid": "c1", "match": "0.9"},
+    ]}}
+    browser = _FakeBrowser()
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        out = LastfmSource(api_key="k", browser=browser).similar_artists("seed")
+    assert out == [SimilarArtist("c1", "Cand One", 0.9)]
+
+
+def test_similar_artists_coerces_single_object():
+    payload = {"similarartists": {"artist": {"name": "Cand One", "mbid": "c1", "match": "0.9"}}}
+    browser = _FakeBrowser()
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        out = LastfmSource(api_key="k", browser=browser).similar_artists("seed")
+    assert out == [SimilarArtist("c1", "Cand One", 0.9)]
+
+
+def test_similar_artists_error_envelope_returns_empty_list():
+    payload = {"error": 6, "message": "no artist"}
+    browser = _FakeBrowser()
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        out = LastfmSource(api_key="k", browser=browser).similar_artists("seed")
+    assert out == []
+
+
+def test_health_true_on_normal_response():
+    payload = {"similarartists": {"artist": [{"name": "Radiohead", "mbid": "x", "match": "1.0"}]}}
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        assert LastfmSource(api_key="k", browser=_FakeBrowser()).health() is True
+
+
+def test_health_false_on_error_envelope():
+    payload = {"error": 10, "message": "invalid api key"}
+    with patch("lyra_worker.similarity._lastfm.requests.get", return_value=_resp(payload)):
+        assert LastfmSource(api_key="k", browser=_FakeBrowser()).health() is False
+
+
+def test_health_false_on_request_exception():
+    with patch("lyra_worker.similarity._lastfm.requests.get",
+               side_effect=requests.ConnectionError("down")):
+        assert LastfmSource(api_key="k", browser=_FakeBrowser()).health() is False

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)