Write a SpacetimeDB backend module in Rust that implements a ban list enforced on game entry.

TABLES
- admin
  - Struct: Admin
  - private table
  - Fields:
    - identity: Identity (primary key)

- banned
  - Struct: Banned
  - private table
  - Fields:
    - identity: Identity (primary key)

- player
  - Struct: Player
  - public table
  - Fields:
    - identity: Identity (primary key)
    - name: String

REDUCERS
- add_admin(ctx, target: Identity)
  - If ctx.sender is not in the admin table, panic with "not admin"
  - Insert Admin { identity: target } (ignore if already present)

- ban_player(ctx, target: Identity)
  - If ctx.sender is not in the admin table, panic with "not admin"
  - Insert Banned { identity: target }
  - Remove the Player row for target if one exists

- join_game(ctx, name: String)
  - If ctx.sender is in the banned table, panic with "banned"
  - Insert Player { identity: ctx.sender, name }; if already present, panic with "already in game"
