- 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