Write a SpacetimeDB backend module in TypeScript that defines one table with a multi-column B-Tree index and seeds rows.

TABLE
- log
  - Fields:
    - id: number (u64, primary key, autoInc)
    - userId: number (i32)
    - day: number (i32)
    - message: string
  - Index: byUserDay (btree on userId, day)

REDUCERS
- seed: insert three rows (IDs are auto-assigned):
  - (userId=7, day=1, message="a")
  - (userId=7, day=2, message="b")
  - (userId=9, day=1, message="c")