Write a SpacetimeDB backend module in Rust 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: u64 (primary key, auto_inc)
    - name: String

- post
  - Struct: Post
  - Fields:
    - id: u64 (primary key, auto_inc)
    - author_id: u64 (index btree)
    - title: String

REDUCERS
- delete_author(ctx, author_id: u64)
  - First, iterate all posts with matching author_id and delete each one
  - Then delete the author by id
