feat(discovery): ListenBrainz similarity source

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_listenbrainz.py

@@ -0,0 +1,43 @@
+from unittest.mock import MagicMock, patch
+
+import requests
+
+from lyra_worker.similarity._listenbrainz import ListenBrainzSource
+from lyra_worker.similarity.base import SimilarArtist
+
+
+def _resp(payload, status=200):
+    r = MagicMock()
+    r.status_code = status
+    r.json.return_value = payload
+    r.raise_for_status.side_effect = (
+        None if status < 400 else requests.HTTPError(f"{status}"))
+    return r
+
+
+def test_similar_artists_parses_and_drops_self():
+    payload = [
+        {"artist_mbid": "seed", "name": "Seed", "score": 100},
+        {"artist_mbid": "c1", "name": "Cand One", "score": 42},
+        {"artist_mbid": "c2", "name": "Cand Two", "score": 7},
+    ]
+    with patch("lyra_worker.similarity._listenbrainz.requests.get", return_value=_resp(payload)):
+        out = ListenBrainzSource().similar_artists("seed")
+    assert out == [SimilarArtist("c1", "Cand One", 42.0), SimilarArtist("c2", "Cand Two", 7.0)]
+
+
+def test_similar_artists_raises_on_http_error():
+    with patch("lyra_worker.similarity._listenbrainz.requests.get", return_value=_resp([], status=500)):
+        try:
+            ListenBrainzSource().similar_artists("seed")
+            assert False, "expected HTTPError"
+        except requests.HTTPError:
+            pass
+
+
+def test_health_true_on_2xx_false_on_exception():
+    with patch("lyra_worker.similarity._listenbrainz.requests.get", return_value=_resp([], status=200)):
+        assert ListenBrainzSource().health() is True
+    with patch("lyra_worker.similarity._listenbrainz.requests.get",
+               side_effect=requests.ConnectionError("down")):
+        assert ListenBrainzSource().health() is False