Add IGDBService rate-limiter unit test

The IGDB service acquires a rate-limiter slot before each request like the
other metadata services, but unlike them had no direct unit test asserting it
(IGDB is otherwise exercised via cassette-backed handler tests). Add a focused
test so the limiter call can't be silently removed or moved after the request.

https://claude.ai/code/session_01133QQuWvq8Zm25DZMP9PVr

backend/tests/adapters/services/test_igdb.py

@@ -0,0 +1,37 @@
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from adapters.services.igdb import IGDBService
+
+
+class TestIGDBServiceUnit:
+    """Unit tests with mocked dependencies."""
+
+    @pytest.fixture
+    def service(self):
+        """Create an IGDBService instance for testing."""
+        return IGDBService(twitch_auth=MagicMock())
+
+    @pytest.mark.asyncio
+    async def test_request_acquires_rate_limiter(self, service, monkeypatch):
+        """Test that the request reserves a rate-limiter slot before sending."""
+        acquire_mock = AsyncMock()
+        monkeypatch.setattr(
+            "adapters.services.igdb._rate_limiter.acquire", acquire_mock
+        )
+
+        mock_session = AsyncMock()
+        mock_response = MagicMock()
+        mock_response.json = AsyncMock(return_value=[{"id": 1}])
+        mock_response.raise_for_status.return_value = None
+        mock_session.post.return_value = mock_response
+
+        mock_context = MagicMock()
+        mock_context.get.return_value = mock_session
+
+        with patch("adapters.services.igdb.ctx_aiohttp_session", mock_context):
+            result = await service._request("https://api.igdb.com/v4/games")
+
+        acquire_mock.assert_awaited_once()
+        assert result == [{"id": 1}]