Write a SpacetimeDB backend module in TypeScript that defines a table, a non-reducer helper function, and a reducer that uses the helper.

TABLE
- result
  - Fields:
    - id: number (i32, primary key)
    - sum: number (i32)

HELPER FUNCTION
- add(a: number, b: number): number - returns a + b

REDUCERS
- computeSum: given id:number, a:number, b:number, insert row with sum=add(a,b)