Replace 3 stub RPCs with real implementations: - get_system_status: fan-out gRPC health checks to all backends, reports SystemStatus - get_health_check: fan-out checks with response time measurement, returns HealthCheck entries - get_metrics: queries Prometheus for requested metric names MonitoringServiceHandler now accepts backend URLs via with_backends() builder. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>