fix: UID 类型混用统一（uint ↔ string）

- Comment.ReplyToUID string→uint, 移除 varchar(36) gorm 约束
- CommentMention.UID string→uint, 移除 varchar(36) gorm 约束
- UserSearchResult.UID string→uint
- 移除 SearchUsersByLevel 中 strconv.FormatUint 转换
- 删除 GetUserUIDByID 方法，CreateReply 直接使用 parent.UserID
- fillReplyToNames 使用 map[uint]string 替代 map[string]string
- 更新 commentStore 接口声明
- 修复前端 mention 下拉 show.html 中 u.id→u.uid

internal/service/comment_service.go

@@ -88,15 +88,12 @@ func (s *CommentService) CreateReply(userID uint, parentID uint, body string) (*
 	}
 
 	comment := &model.Comment{
-		PostID:   parent.PostID,
-		RootID:   parent.RootID,
-		ParentID: &parentID,
-		Body:     body,
-		UserID:   userID,
-	}
-	// 获取被回复者UID
-	if replyToUID, err := s.repo.GetUserUIDByID(parent.UserID); err == nil {
-		comment.ReplyToUID = replyToUID
+		PostID:     parent.PostID,
+		RootID:     parent.RootID,
+		ParentID:   &parentID,
+		Body:       body,
+		UserID:     userID,
+		ReplyToUID: parent.UserID,
 	}
 
 	if err := s.repo.Create(comment); err != nil {

internal/service/repository.go

@@ -154,11 +154,10 @@ type commentStore interface {
 	FindPostIDByComment(commentID uint) (uint, error)
 	FindPostAuthorID(postID uint) (uint, error)
 	SearchUsersByLevel(keyword string, minLevel, limit int) ([]struct {
-		UID      string
+		UID      uint
 		Username string
 		Exp      int
 	}, error)
-	GetUserUIDByID(userID uint) (string, error)
 	GetUsernameByID(userID uint) (string, error)
 	ListAllComments(keyword string, showDeleted bool, offset, limit int) ([]model.AdminCommentRow, int64, error)
 }