Write a SpacetimeDB backend module in C# that defines an Author table and a Post table with a foreign key, and a reducer that deletes an author along with all their posts.

TABLES
- Author
  - Struct: Author
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - Name: string

- Post
  - Struct: Post
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - AuthorId: ulong (index BTree)
    - Title: string

REDUCERS
- DeleteAuthor(ctx, ulong authorId)
  - First, iterate all posts with matching AuthorId and delete each one
  - Then delete the author by Id
