feat: NotificationService에 사용자 모든 알림 삭제 기능 추가

Author: MEOHIN

src/main/java/com/back/domain/notification/service/NotificationService.java

@@ -115,4 +115,9 @@ public void sendNotification(User user, Post post, NotificationType type, String
         }
       }
     }
+
+    @Transactional
+    public void deleteAll(Long userId) {
+        notificationRepository.deleteByUser_Id(userId);
+    }
 }