fix: resolve schemathesis and live test failures

- Add 403 response to all RBAC-gated endpoints (schemathesis UndefinedStatusCode)
- Add 400 response to all endpoints accepting JSON bodies (malformed input)
- Add required 'title' field to schemathesis.toml for schemathesis 4.15+
- Add xdist_group markers to live tests with module-scoped fixtures to
  prevent xdist from distributing them across workers (fixture isolation)

tests/live/test_health_live.py

@@ -97,6 +97,7 @@ async def token(live_client):
 
 
 @pytest.mark.live
+@pytest.mark.xdist_group("health_live")
 class TestHealthLive:
     """Live integration tests — real DB, real Docker check, real task state."""