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

TABLES
- Message
  - Struct: Message
  - Fields:
    - Id: ulong (primary key, AutoInc)
    - Owner: Identity (index BTree)
    - Text: string

REDUCERS
- CreateMessage(ctx, string text)
  - Insert a new Message with Owner set to ctx.Sender
  - Use Id: 0 for AutoInc

- DeleteMessage(ctx, ulong id)
  - Find the message by Id
  - If not found, throw exception with "not found"
  - If the message's Owner does not equal ctx.Sender, throw exception with "unauthorized"
  - Otherwise, delete the message
