Write a SpacetimeDB backend module in TypeScript that demonstrates multi-column B-Tree indexes.

TABLE
- log
  - Fields:
    - id: number (i32, primary key)
    - userId: number (i32)
    - day: number (i32)
    - message: string
  - Index: byUserDay (btree on userId, day)

REDUCERS
- seed: insert three rows:
  - (id=1, userId=7, day=1, message="a")
  - (id=2, userId=7, day=2, message="b")
  - (id=3, userId=9, day=1, message="c")
