Upgrade the Locust load-test dependency to 2.47.0 #2475

Problem

EventRelay PR #2464 updates the Locust requirement to 2.47.0 while the previous uv.lock still resolved 2.46.0, so the Python CI lock check failed.

Completion criteria

• Keep pyproject.toml and uv.lock synchronized with the CI-pinned uv version, including Locust's required transitive dependencies.
• Confirm the repository's tests/load/locustfile.py imports and registers EventRelayUser under Locust 2.47.0; the app does not call the changed /stop or /stats/reset Locust API routes.
• Verify the exact PR head's required checks and review findings before merge.

Tracked by #2464. Merging the completed PR should close this issue.