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

worker/lyra_worker/similarity/_lastfm.py

@@ -0,0 +1,69 @@
+import requests
+
+from lyra_worker.similarity.base import SimilarArtist
+
+_BASE = "https://ws.audioscrobbler.com/2.0/"
+
+
+class LastfmSource:
+    """SimilaritySource backed by Last.fm's artist.getsimilar.
+
+    Last.fm rows often carry an empty mbid, so names are resolved to a real
+    MusicBrainz artist MBID via the injected browser. Unresolvable rows are
+    skipped -- every emitted SimilarArtist.mbid must be a real MB artist MBID,
+    since downstream keys on it.
+    """
+
+    name = "lastfm"
+
+    def __init__(self, api_key, browser, base_url=_BASE, timeout: float = 15.0,
+                 similar_limit: int = 50):
+        self._key = api_key
+        self._browser = browser  # MbBrowser: .search_artist(name) -> list[ArtistHit]
+        self._base = base_url
+        self._timeout = timeout
+        self._limit = similar_limit
+        self._name_cache: dict[str, str | None] = {}
+
+    def _get(self, method: str, **params):
+        res = requests.get(
+            self._base,
+            params={"method": method, "api_key": self._key, "format": "json", **params},
+            timeout=self._timeout,
+        )
+        return res.json()
+
+    def health(self) -> bool:
+        try:
+            data = self._get("artist.getsimilar", artist="Radiohead", limit="1")
+            return not isinstance(data.get("error"), (int, float))
+        except requests.RequestException:
+            return False
+
+    def _resolve(self, name: str) -> str | None:
+        key = name.lower()
+        if key in self._name_cache:
+            return self._name_cache[key]
+        hits = self._browser.search_artist(name)
+        mbid = hits[0].mbid if hits else None
+        self._name_cache[key] = mbid
+        return mbid
+
+    def similar_artists(self, mbid: str) -> list[SimilarArtist]:
+        data = self._get("artist.getsimilar", mbid=mbid, limit=str(self._limit))
+        if isinstance(data.get("error"), (int, float)):
+            return []
+        rows = data.get("similarartists", {}).get("artist", [])
+        if isinstance(rows, dict):
+            rows = [rows]
+        out: list[SimilarArtist] = []
+        for row in rows:
+            name = row.get("name") or ""
+            cand = row.get("mbid") or None
+            if not cand:
+                cand = self._resolve(name)
+            if not cand or cand == mbid or not name:
+                continue
+            score = float(row.get("match") or 0)
+            out.append(SimilarArtist(mbid=cand, name=name, score=score))
+        return out

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)