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

TABLES
- order
  - Struct: Order
  - Fields:
    - id: u64 (primary key, auto_inc)
    - category: String (index btree)
    - amount: u64
    - fulfilled: bool

- category_stats
  - Struct: CategoryStats
  - Fields:
    - category: String (primary key)
    - total_amount: u64
    - order_count: 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
