Add the persistence layer for the checker system:
- Storage interfaces (CheckPlanStorage, CheckerOptionsStorage,
CheckEvaluationStorage, ExecutionStorage, ObservationSnapshotStorage,
SchedulerStateStorage) in the usecase/checker package
- KV-based implementations for LevelDB/Oracle NoSQL/InMemory backends
- Integrate checker storage into the main Storage interface
- Add tidy methods for checker entities (plans, configurations,
evaluations, executions, snapshots, observation cache) and
secondary index cleanup
