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

TABLE
- reminder
  - Fields:
    - scheduledId: u64 (primary key, auto-increment)
    - scheduledAt: scheduleAt
    - message: string
  - Scheduling: scheduled to call the send_reminder reducer

REDUCERS
- send_reminder: scheduled reducer triggered by reminder (receives a reminder row argument, does nothing)
- init: inserts one row into reminder scheduling send_reminder to fire 60 seconds in the future using ScheduleAt.time
  - Use ctx.timestamp + 60_000_000n microseconds to compute the fire time as a BigInt
