Write a SpacetimeDB backend module in Rust that defines three tables modeling a many-to-many relationship and seeds rows.

TABLES
- user
  - Struct: User
  - Fields:
    - user_id: u64 (primary key, auto_inc)
    - name: String

- group
  - Struct: Group
  - Fields:
    - group_id: u64 (primary key, auto_inc)
    - title: String

- membership
  - Struct: Membership
  - Fields:
    - id: u64 (primary key, auto_inc)
    - user_id: u64
    - group_id: u64
  - Indexes:
    - by_user: btree(user_id)
    - by_group: btree(group_id)

REDUCERS
- seed: insert exactly these rows (IDs are auto-assigned)
  - user: (name="Alice"), (name="Bob")
  - group: (title="Admin"), (title="Dev")
  - membership:
    - (user_id=1, group_id=1)
    - (user_id=1, group_id=2)
    - (user_id=2, group_id=2)
