﻿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: i32 (primary key)
    - name: String

- group
  - Struct: Group
  - Fields:
    - group_id: i32 (primary key)
    - title: String

- membership
  - Struct: Membership
  - Fields:
    - id: i32 (primary key)
    - user_id: i32
    - group_id: i32
  - Indexes:
    - by_user: btree(user_id)
    - by_group: btree(group_id)

REDUCERS
- seed: insert exactly these rows
  - user: (user_id=1, name="Alice"), (user_id=2, name="Bob")
  - group: (group_id=10, title="Admin"), (group_id=20, title="Dev")
  - membership:
    - (id=1, user_id=1, group_id=10)
    - (id=2, user_id=1, group_id=20)
    - (id=3, user_id=2, group_id=20)
