Write a SpacetimeDB backend module in C# that defines one table and a reducer that inserts a row.

TABLE
- User
  - Struct: User
  - Fields:
    - Id: int (primary key)
    - Name: string
    - Age: int
    - Active: bool

REDUCERS
- InsertUser: given id:int, name:string, age:int, active:bool, insert exactly one row into User
  - (Id=id, Name=name, Age=age, Active=active)
