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

TABLES
- message
  - Struct: Message
  - public table
  - Fields:
    - id: u64 (primary key, auto_inc)
    - owner: Identity (index btree)
    - text: String

REDUCERS
- create_message(ctx, text: String)
  - Insert a new Message with owner set to ctx.sender
  - Use id: 0 for auto_inc

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