feat: retry MusicBrainz barcode lookup with random delay on empty result

Up to 3 retries with 2–6 s random wait between attempts, as MusicBrainz
occasionally returns no results on the first try. retries parameter is
configurable (default: 3).

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

tests/test_musicbrainz.py

@@ -171,9 +171,42 @@ class TestLookupByBarcode:
         with (
             patch("musiksammlung.musicbrainz.httpx.get", return_value=empty),
             patch("musiksammlung.musicbrainz.time.sleep"),
+            patch("musiksammlung.musicbrainz.random.uniform", return_value=0.0),
             pytest.raises(ValueError, match="Kein MusicBrainz-Eintrag"),
         ):
-            lookup_by_barcode("0000000000000")
+            lookup_by_barcode("0000000000000", retries=0)
+
+    def test_retries_on_empty_result(self) -> None:
+        """Bei leerem Ergebnis wird bis zu retries-mal wiederholt."""
+        empty = _mock_response({"releases": []})
+        hit = _mock_response(_BARCODE_RESPONSE)
+        # Erste zwei Versuche leer, dritter Versuch Treffer
+        responses = [empty, empty, hit, _mock_response(_RELEASE_RESPONSE)]
+        with (
+            patch("musiksammlung.musicbrainz.httpx.get", side_effect=responses) as mock_get,
+            patch("musiksammlung.musicbrainz.time.sleep"),
+            patch("musiksammlung.musicbrainz.random.uniform", return_value=0.0),
+        ):
+            album = lookup_by_barcode("0602557360561", retries=3)
+
+        assert album.artist == "The Beatles"
+        # 3 Barcode-Requests + 1 Release-Request
+        assert mock_get.call_count == 4
+
+    def test_retry_sleep_is_called_between_attempts(self) -> None:
+        """Zwischen den Versuchen wird time.sleep aufgerufen."""
+        empty = _mock_response({"releases": []})
+        responses = [empty, _mock_response(_BARCODE_RESPONSE), _mock_response(_RELEASE_RESPONSE)]
+        with (
+            patch("musiksammlung.musicbrainz.httpx.get", side_effect=responses),
+            patch("musiksammlung.musicbrainz.time.sleep") as mock_sleep,
+            patch("musiksammlung.musicbrainz.random.uniform", return_value=2.5),
+        ):
+            lookup_by_barcode("0602557360561", retries=2)
+
+        # Einmal Retry-Sleep (2.5 s) + einmal Rate-Limit-Sleep (≥1.1 s)
+        assert mock_sleep.call_count == 2
+        assert mock_sleep.call_args_list[0][0][0] == 2.5
 
     def test_uses_first_release(self) -> None:
         barcode_data = {"releases": [{"id": "first-id"}, {"id": "second-id"}]}