Write a SpacetimeDB backend module in Rust that defines an event table for damage notifications and a reducer that publishes events to it.

TABLES
- damage_event
  - Struct: DamageEvent
  - public event table (rows exist only for the duration of the transaction)
  - Fields:
    - entity_id: u64
    - damage: u32
    - source: String

REDUCERS
- deal_damage(ctx, entity_id: u64, damage: u32, source: String)
  - Insert a DamageEvent row into the event table
