fix: address review feedback

- Removed duplicate blank line in event.go
- Use original event timestamp instead of time.Now() for EventStored
- Fixed MockEventBroadcaster.Subscribe to return nil instead of closed channel
- Added integration tests for EventStored with JetStreamEventStore

Co-Authored-By: Claude Code <noreply@anthropic.com>

store/jetstream.go

@@ -235,7 +235,7 @@ func (jes *JetStreamEventStore) publishEventStored(event *aether.Event) {
 		EventID:   event.ID,
 		ActorID:   event.ActorID,
 		Version:   event.Version,
-		Timestamp: time.Now(),
+		Timestamp: event.Timestamp,
 	}
 
 	// Convert EventStored to Event for publishing (internal system event)