fix: 已锁定文章禁止提交审核，隐藏提交按钮

- SubmitForAudit 新增 IsLocked 检查，锁定文章不可提交审核
- posts/show.html：已锁定文章不显示"提交审核"按钮

internal/service/post_service.go

@@ -251,6 +251,9 @@ func (s *PostService) SubmitForAudit(postID uint) error {
 	if err != nil {
 		return err
 	}
+	if post.IsLocked {
+		return common.ErrPostCannotEdit
+	}
 	if post.Status != model.PostStatusDraft && post.Status != model.PostStatusRejected {
 		return common.ErrPostCannotSubmit
 	}