feat(NotificationHandlerImpl): 트랜잭션 추가

- DB save 작업

Author: m-a-king

src/main/java/com/somemore/notification/handler/NotificationHandlerImpl.java

@@ -8,9 +8,11 @@
 import com.somemore.sse.usecase.SseUseCase;
 import lombok.RequiredArgsConstructor;
 import org.springframework.stereotype.Component;
+import org.springframework.transaction.annotation.Transactional;
 
 @Component
 @RequiredArgsConstructor
+@Transactional
 public class NotificationHandlerImpl implements NotificationHandler {
 
     private final NotificationRepository notificationRepository;