Add server-backed file property filters

tests/test_router_contract.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import unittest
 from collections import Counter
+from inspect import signature
 
 from govoplan_files.backend.router import router
 from govoplan_files.backend.routes.assets import router as assets_router
@@ -76,6 +77,18 @@ class FilesRouterContractTests(unittest.TestCase):
         self.assertIn((("POST",), "/files/bulk-rename"), routes)
         self.assertIn((("POST",), "/files/transfer"), routes)
 
+    def test_file_listing_exposes_structured_property_filters(self) -> None:
+        route = next(
+            route
+            for route in listing_router.routes
+            if route.path == "/files" and "GET" in (route.methods or ())
+        )
+        parameters = signature(route.endpoint).parameters
+
+        self.assertIn("campaign_usage", parameters)
+        self.assertIn("audit_relevant", parameters)
+        self.assertIn("total", route.response_model.model_fields)
+
 
 if __name__ == "__main__":
     unittest.main()