feat: 文章锁定/解锁时触发系统通知

- 新增 NotifyPostLocked/NotifyPostUnlocked 通知类型
- Lock/Unlock 方法执行后通过 notifier 发送通知给作者
- 锁定时通知含锁定理由，解锁时通知已解除锁定
- 新增类型归入稿件审核分类和 system tab

internal/service/repository.go

@@ -75,6 +75,8 @@ type userExpReader interface {
 type postNotifier interface {
 	NotifyPostApproved(userID uint, postTitle string, postID uint)
 	NotifyPostRejected(userID uint, postTitle, reason string, postID uint)
+	NotifyPostLocked(userID uint, postTitle, reason string, postID uint)
+	NotifyPostUnlocked(userID uint, postTitle string, postID uint)
 }
 
 // userLevelStore LevelService 所需的用户仓储接口