Write a SpacetimeDB backend module in TypeScript that defines an order table and a stats table, with a reducer that computes aggregate statistics from filtered orders.

TABLES
- order
  - Fields:
    - id: number (u64, primary key, autoInc)
    - category: string (index btree)
    - amount: number (u64)
    - fulfilled: boolean

- categoryStats
  - Fields:
    - category: string (primary key)
    - totalAmount: number (u64)
    - orderCount: number (u32)

REDUCERS
- compute_stats(ctx, { category: string })
  - Filter all orders matching the given category
  - Count the number of matching orders and sum their amounts
  - Insert (or update if exists) a categoryStats row with the results
