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

TABLES
- user
  - Fields:
    - userId: number (u64, primary key, autoInc)
    - name: string

- group
  - Fields:
    - groupId: number (u64, primary key, autoInc)
    - title: string

- membership
  - Fields:
    - id: number (u64, primary key, autoInc)
    - userId: number (u64)
    - groupId: number (u64)
  - Indexes:
    - byUser (btree on userId)
    - byGroup (btree on groupId)

REDUCERS
- seed: insert users, groups, and memberships (IDs are auto-assigned):
  - user: ("Alice"), ("Bob")
  - group: ("Admin"), ("Dev")
  - membership: (userId=1, groupId=1), (userId=1, groupId=2), (userId=2, groupId=2)