Write a SpacetimeDB backend module in C# that defines an Order table and a CategoryStats table, with a reducer that computes aggregate statistics from filtered orders.

TABLES
- Order
  - Struct: Order
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - Category: string (index BTree)
    - Amount: ulong
    - Fulfilled: bool

- CategoryStats
  - Struct: CategoryStats
  - Fields:
    - Category: string (primary key)
    - TotalAmount: ulong
    - OrderCount: uint

REDUCERS
- ComputeStats(ctx, string category)
  - 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
