Write a SpacetimeDB backend module in TypeScript that defines a table of messages owned by users, and reducers that enforce ownership using ctx.sender.

TABLES
- message
  - public table
  - Fields:
    - id: number (u64, primary key, autoInc)
    - owner: Identity (index btree)
    - text: string

REDUCERS
- create_message(ctx, { text: string })
  - Insert a new message with owner set to ctx.sender()
  - Use id: 0n for autoInc

- delete_message(ctx, { id: number (u64) })
  - Find the message by id
  - If not found, throw "not found"
  - If the message's owner does not equal ctx.sender(), throw "unauthorized"
  - Otherwise, delete the message
