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: ulong (primary key, auto-increment)
    - Name: string

- Group
  - Struct: Group
  - Fields:
    - GroupId: ulong (primary key, auto-increment)
    - Title: string

- Membership
  - Struct: Membership
  - Fields:
    - Id: ulong (primary key, auto-increment)
    - UserId: ulong
    - GroupId: ulong
  - Indexes:
    - by_user: btree(UserId)
    - by_group: btree(GroupId)

REDUCERS
- Seed: insert exactly these rows (IDs are auto-assigned)
  - User: (Name="Alice"), (Name="Bob")
  - Group: (Title="Admin"), (Title="Dev")
  - Membership:
    - (UserId=1, GroupId=1)
    - (UserId=1, GroupId=2)
    - (UserId=2, GroupId=2)
