Write a SpacetimeDB backend module in TypeScript that uses sum types in multiple table columns.

TYPES
- Rect
  - width: number (i32)
  - height: number (i32)
- Shape (sum type / enum)
  - circle: number (i32 radius)
  - rectangle: Rect

TABLE
- drawing
  - Fields:
    - id: number (i32, primary key)
    - a: Shape
    - b: Shape

REDUCERS
- seed: insert one row with two Shape values:
  - a: Shape.circle(10)
  - b: Shape.rectangle({ width: 4, height: 6 })
