- Add HealthStatus enum with healthy/degraded/unhealthy states and HealthCheck struct with name/status/message/last_checked/response_time/metadata fields
- Add SystemHealth struct with status/timestamp/uptime/version/environment/checks/summary fields and HealthSummary with total/healthy/degraded/unhealthy counts
- Add HealthMonitor struct with checks map, mutex, startTime, version, env fields