Deduplicate dependency version pins from backend code/tests

backend/tests/test_basic.py

@@ -98,15 +98,10 @@ def test_compatibility_status_success(monkeypatch: pytest.MonkeyPatch):
     """Compatibility status should be healthy when all checks match policy."""
 
     monkeypatch.setenv("BACKEND_REQUIRED_PYTHON", "3.14")
+    required_pins = cast("dict[str, str]", compatibility_status()["required_packages"])
 
     def installed_version(package_name: str) -> str:
-        versions = {
-            "fastapi": "0.139.0",
-            "psycopg": "3.2.12",
-            "sqlalchemy": "2.0.44",
-            "uvicorn": "0.51.0",
-        }
-        return versions[package_name]
+        return required_pins[package_name]
 
     monkeypatch.setattr("backend.main._installed_version", installed_version)
     status = cast("dict[str, Any]", compatibility_status())