feat(lidarr): MusicBrainz track-to-album resolver

Add musicbrainz_best_album() that resolves an artist+track pair to its
best studio album via the MusicBrainz search API, with a 1 req/sec
courtesy rate-limiter. Prefers plain studio albums over compilations,
singles, and live releases; falls back to any release group when no
studio album is found. Never raises — returns None on any failure.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_musicbrainz.py

@@ -0,0 +1,79 @@
+import server.mf  # noqa: F401
+import musicfetch_core as mf
+
+
+class _FakeResp:
+    def __init__(self, payload):
+        self._payload = payload
+    def raise_for_status(self):
+        pass
+    def json(self):
+        return self._payload
+
+
+MB_PAYLOAD = {
+    "recordings": [
+        {
+            "artist-credit": [{"name": "Daft Punk"}],
+            "releases": [
+                {"date": "2001",
+                 "release-group": {"id": "single-mbid", "title": "Harder, Better, Faster, Stronger",
+                                   "primary-type": "Single", "secondary-types": []}},
+                {"date": "2002",
+                 "release-group": {"id": "comp-mbid", "title": "Musique, Vol. 1",
+                                   "primary-type": "Album", "secondary-types": ["Compilation"]}},
+                {"date": "2001",
+                 "release-group": {"id": "48117b90-a16e-34ca-a514-19c702df1158",
+                                   "title": "Discovery", "primary-type": "Album",
+                                   "secondary-types": []}},
+            ],
+        }
+    ]
+}
+
+
+def test_picks_studio_album_over_single_and_comp(monkeypatch):
+    monkeypatch.setattr(mf.requests, "get", lambda *a, **k: _FakeResp(MB_PAYLOAD))
+    monkeypatch.setattr(mf.time, "sleep", lambda *_: None)
+    out = mf.musicbrainz_best_album("Daft Punk", "Harder Better Faster Stronger")
+    assert out["album_title"] == "Discovery"
+    assert out["artist"] == "Daft Punk"
+    assert out["year"] == "2001"
+    assert out["rg_mbid"] == "48117b90-a16e-34ca-a514-19c702df1158"
+
+
+def test_returns_none_on_empty(monkeypatch):
+    monkeypatch.setattr(mf.requests, "get", lambda *a, **k: _FakeResp({"recordings": []}))
+    monkeypatch.setattr(mf.time, "sleep", lambda *_: None)
+    assert mf.musicbrainz_best_album("Nobody", "Nothing") is None
+
+
+def test_returns_none_on_exception(monkeypatch):
+    def boom(*a, **k):
+        raise mf.requests.exceptions.RequestException("network down")
+    monkeypatch.setattr(mf.requests, "get", boom)
+    monkeypatch.setattr(mf.time, "sleep", lambda *_: None)
+    assert mf.musicbrainz_best_album("Daft Punk", "Discovery") is None
+
+
+def test_falls_back_to_any_releasegroup_when_no_studio(monkeypatch):
+    payload = {"recordings": [{"artist-credit": [{"name": "X"}], "releases": [
+        {"date": "2010", "release-group": {"id": "live1", "title": "Live Thing",
+                                           "primary-type": "Album", "secondary-types": ["Live"]}},
+    ]}]}
+    monkeypatch.setattr(mf.requests, "get", lambda *a, **k: _FakeResp(payload))
+    monkeypatch.setattr(mf.time, "sleep", lambda *_: None)
+    out = mf.musicbrainz_best_album("X", "Y")
+    assert out["album_title"] == "Live Thing"
+
+
+def test_first_artist_credit_only(monkeypatch):
+    payload = {"recordings": [{"artist-credit": [{"name": "SLVMLORD"}, {"name": "Travis Bradley"}],
+                              "releases": [{"date": "2025",
+                                            "release-group": {"id": "x", "title": "Album X",
+                                                              "primary-type": "Album",
+                                                              "secondary-types": []}}]}]}
+    monkeypatch.setattr(mf.requests, "get", lambda *a, **k: _FakeResp(payload))
+    monkeypatch.setattr(mf.time, "sleep", lambda *_: None)
+    out = mf.musicbrainz_best_album("SLVMLORD", "Under My Skin")
+    assert out["artist"] == "SLVMLORD"