feat: 锁定文章需填写锁定理由，编辑器显示锁定理由

- Post 模型新增 LockReason 字段，锁定必填理由，解锁清空
- PostLockRequest DTO：reason 必填，1-500字
- Admin JS：锁定操作弹出 prompt 要求填写理由
- 编辑器锁定横幅显示"锁定理由：XXX"，理由为空时不显示该行

internal/controller/admin/admin_post_controller.go

@@ -116,7 +116,13 @@ func (ctrl *AdminPostController) Lock(c *gin.Context) {
 		return
 	}
 
-	if err := ctrl.postService.Lock(uint(id)); err != nil {
+	var req model.PostLockRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		common.Error(c, http.StatusBadRequest, "请填写锁定理由")
+		return
+	}
+
+	if err := ctrl.postService.Lock(uint(id), req.Reason); err != nil {
 		if errors.Is(err, common.ErrPostNotFound) || errors.Is(err, common.ErrPostCannotLock) {
 			common.Error(c, http.StatusBadRequest, err.Error())
 		} else {

internal/controller/admin/interfaces.go

@@ -42,7 +42,7 @@ type adminPostUseCase interface {
 	ListPendingRevisions(keyword string, page, pageSize int) ([]model.Post, int64, error)
 	Approve(postID uint) error
 	Reject(postID uint, reason string) error
-	Lock(postID uint) error
+	Lock(postID uint, reason string) error
 	Unlock(postID uint) error
 	Restore(postID uint) error
 }