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

TABLE
- Log
  - Struct: Log
  - Fields:
    - Id: int (primary key)
    - UserId: int
    - Day: int
    - Message: string
  - Indexes:
    - by_user_day: btree(UserId, Day)

REDUCERS
- Seed: insert exactly these rows into Log
  - (Id=1, UserId=7, Day=1, Message="a")
  - (Id=2, UserId=7, Day=2, Message="b")
  - (Id=3, UserId=9, Day=1, Message="c")
