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

TABLE
- reminder
  - Struct: Reminder
  - Fields:
    - scheduled_id: u64 (primary key, auto-increment)
    - scheduled_at: ScheduleAt
    - message: String
  - Scheduling:
    - reducer: send_reminder

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