- Add health.rs module with start_health_check_listener() that binds to
127.0.0.1:{health_check_port} and serves GET /health returning 200 OK
with empty body
- Add health_route() in proxy/handler.rs for HTTPS listener fallback
- Add port conflict detection in config validation: health_check_port
must not conflict with listener ports on 127.0.0.1/localhost/0.0.0.0
- health_check_port = 0 disables the separate listener (handled at call
site)
- Add unit and integration tests for health check functionality