Write a SpacetimeDB backend module in Rust that schedules a reducer and allows canceling it before it fires.

TABLE
- cleanup_job
  - Struct: CleanupJob
  - Fields:
    - scheduled_id: u64 (primary key, auto-increment)
    - scheduled_at: ScheduleAt
  - Scheduling:
    - reducer: run_cleanup

REDUCERS
- run_cleanup: scheduled reducer triggered by cleanup_job (receives a CleanupJob row, does nothing)
- init: inserts one row into cleanup_job with a 60-second interval
- cancel_cleanup(ctx, scheduled_id: u64): deletes the CleanupJob row with the given scheduled_id, canceling the pending scheduled call
