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

TABLES
- User
  - Struct: User
  - Fields:
    - UserId: int (primary key)
    - Name: string

- Group
  - Struct: Group
  - Fields:
    - GroupId: int (primary key)
    - Title: string

- Membership
  - Struct: Membership
  - Fields:
    - Id: int (primary key)
    - UserId: int
    - GroupId: int
  - Indexes:
    - by_user: btree(UserId)
    - by_group: btree(GroupId)

REDUCERS
- Seed: insert exactly these rows
  - User: (UserId=1, Name="Alice"), (UserId=2, Name="Bob")
  - Group: (GroupId=10, Title="Admin"), (GroupId=20, Title="Dev")
  - Membership:
    - (Id=1, UserId=1, GroupId=10)
    - (Id=2, UserId=1, GroupId=20)
    - (Id=3, UserId=2, GroupId=20)
