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

TABLE
- log
  - Struct: Log
  - Fields:
    - id: i32 (primary key)
    - user_id: i32
    - day: i32
    - message: String
  - Indexes:
    - by_user_day: btree(user_id, day)

REDUCERS
- seed: insert exactly these rows into log
  - (id=1, user_id=7, day=1, message="a")
  - (id=2, user_id=7, day=2, message="b")
  - (id=3, user_id=9, day=1, message="c")
