Implement the checker business logic:
- CheckerOptionsUsecase: scope-based option resolution, validation,
auto-fill from execution context (domain, zone, service)
- CheckPlanUsecase: CRUD for user scheduling configurations
- CheckStatusUsecase: aggregated status queries, execution history
- CheckerEngine: full execution pipeline (observe, evaluate, aggregate)
- Scheduler: background job executor with auto-discovery, min-heap
queue, worker pool, and jitter-based scheduling