Write a SpacetimeDB backend module in Rust 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: u64 (primary key, auto_inc)
    - category: String
    - amount: u32

- distinct_category
  - Struct: DistinctCategory
  - Fields:
    - category: String (primary key)

REDUCERS
- collect_distinct_categories(ctx)
  - Iterate all orders
  - Collect unique category values (use a HashSet or BTreeSet)
  - Insert each unique category into distinct_category
