replace tokio::broadcast event bus with SQLite-backed event queue (ADR-0003)

- DomainEvent: add Serialize/Deserialize
- EventConsumer port: poll_next/ack/nack replacing recv()
- EventEnvelope: wraps event with id/retry_count/created_at
- SqliteEventPublisher/SqliteEventConsumer: INSERT/poll/DLQ
- migration: event_queue + dead_letter_queue tables
- InMemoryEventBus: combined publisher+consumer test double
- NoopEventConsumer: test stub
- webhook_consumer: polls EventConsumer instead of broadcast::Receiver
- presentation+mcp wiring: create from SqlitePool

Cargo.lock

@@ -34,7 +34,11 @@ name = "adapter-event-publisher"
 version = "0.1.0"
 dependencies = [
  "async-trait",
+ "chrono",
  "domain",
+ "serde",
+ "serde_json",
+ "sqlx",
  "tokio",
  "tracing",
 ]