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

crates/mcp/src/main.rs

@@ -44,8 +44,12 @@ async fn main() -> anyhow::Result<()> {
 
     let provider_registry = build_provider_registry().await;
 
-    let event_bus = Arc::new(adapter_event_publisher::ChannelEventBus::new(64));
-    let event_publisher: Arc<dyn domain::ports::EventPublisher> = event_bus.clone();
+    let sqlite_pool = match &pool {
+        #[cfg(feature = "sqlite")]
+        DbPool::Sqlite(p) => p.clone(),
+    };
+    let event_publisher: Arc<dyn domain::ports::EventPublisher> =
+        Arc::new(adapter_event_publisher::SqliteEventPublisher::new(sqlite_pool));
 
     let schedule_engine = Arc::new(ScheduleEngineService::new(
         wire.library_query.clone(),