Write a SpacetimeDB backend module in TypeScript that models many-to-many relationships with a junction table.

TABLES
- user
  - Fields:
    - userId: number (i32, primary key)
    - name: string

- group
  - Fields:
    - groupId: number (i32, primary key)
    - title: string

- membership
  - Fields:
    - id: number (i32, primary key)
    - userId: number (i32)
    - groupId: number (i32)
  - Indexes:
    - byUser (btree on userId)
    - byGroup (btree on groupId)

REDUCERS
- seed: insert users, groups, and memberships:
  - user: (1, "Alice"), (2, "Bob")
  - group: (10, "Admin"), (20, "Dev")
  - membership: (1, userId=1, groupId=10), (2, userId=1, groupId=20), (3, userId=2, groupId=20)
