Add Server-Timing header to streaming endpoint

Reports cache_check latency in the response header, visible in
browser DevTools Network tab for ongoing performance monitoring.

tests/test_listing_geojson.py

@@ -355,4 +355,12 @@ class TestStreamingEndpoint:
             assert 2 not in feature_ids
             assert 1 in feature_ids
 
+    def test_streaming_includes_server_timing_header(self, client, mock_repository):
+        """Test that streaming response includes Server-Timing header."""
+        response = client.get("/api/listing_geojson/stream?listing_type=RENT&limit=10")
+        assert response.status_code == 200
+        assert "server-timing" in response.headers
+        assert "cache_check" in response.headers["server-timing"]
+
+