Information disclosure: health_check exposes aggregate relayer metrics without auth

## Summary

The `health_check` JSON-RPC method returns total, pending, completed, and failed request counts plus uptime (`src/methods/health_check.rs`).

## Risk

On a publicly reachable deployment, this is a low-severity **information leak** useful for reconnaissance (activity levels, failure rates) and capacity planning by competitors or attackers.

## Suggested mitigations

- Require the same authentication as other methods, or bind health to a separate admin port.
- Return coarse-grained status only for authenticated callers.

## References

- `src/methods/health_check.rs`
- `src/rpc.rs` (method registration)