Write a SpacetimeDB backend module in C# that defines three tables with basic columns.

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

- Product
  - Struct: Product
  - Fields:
    - Id: int (primary key)
    - Title: string
    - Price: float
    - InStock: bool

- Note
  - Struct: Note
  - Fields:
    - Id: int (primary key)
    - Body: string
    - Rating: long
    - Pinned: bool
