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

TABLE
- CleanupJob
  - Fields:
    - ScheduledId: ulong (primary key, auto-increment)
    - ScheduledAt: ScheduleAt (the schedule column)
  - Scheduling:
    - Scheduled = nameof(RunCleanup)
    - ScheduledAt = nameof(CleanupJob.ScheduledAt)

REDUCERS
- RunCleanup: scheduled reducer triggered by CleanupJob (receives a CleanupJob row, does nothing)
- Init (ReducerKind.Init): inserts one row into CleanupJob with a 60-second interval (60_000_000 microseconds)
- CancelCleanup(ctx, scheduledId: ulong): deletes the CleanupJob row with the given scheduledId, canceling the pending scheduled call
