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)

decnet/web/router/stream/api_stream_events.py

@@ -50,6 +50,7 @@ def _build_trace_links(logs: list[dict]) -> list:
             "description": "Real-time Server-Sent Events (SSE) stream"
         },
         401: {"description": "Could not validate credentials"},
+        403: {"description": "Insufficient permissions"},
         422: {"description": "Validation error"}
     },
 )