- 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
- Add PrometheusService struct with baseURL/http.Client (10s timeout) and PROMETHEUS_URL env config (default: http://prometheus:9090)
- Add Available method with /-/ready endpoint health check
- Add Query method with instant PromQL execution via /api/v1/query, URL escaping, error handling
- Add promResponse/promVectorData/PromVectorSample structs for query result parsing with metric/value fields