Write a SpacetimeDB backend module in C# that schedules a one-shot reducer to fire at a specific future time.

TABLE
- Reminder
  - Fields:
    - ScheduledId: ulong (primary key, auto-increment)
    - ScheduledAt: ScheduleAt (the schedule column)
    - Message: string
  - Scheduling:
    - Scheduled = nameof(SendReminder)
    - ScheduledAt = nameof(Reminder.ScheduledAt)

REDUCERS
- SendReminder: scheduled reducer triggered by Reminder (receives a Reminder row, does nothing)
- Init (ReducerKind.Init): inserts one row into Reminder using ScheduleAt.Time with a TimeDuration of 60 seconds (60_000_000 microseconds) added to ctx.Timestamp
