Write a SpacetimeDB backend module in C# that defines an Order table and a result table, with a reducer that collects all distinct Category values from orders into the result table.

TABLES
- Order
  - Struct: Order
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - Category: string
    - Amount: uint

- DistinctCategory
  - Struct: DistinctCategory
  - Fields:
    - Category: string (primary key)

REDUCERS
- CollectDistinctCategories(ctx)
  - Iterate all orders
  - Collect unique Category values (use a HashSet)
  - Insert each unique category into DistinctCategory
