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/repository/comment_repo.go

@@ -96,7 +96,7 @@ func (r *CommentRepo) fillReplyToNames(list []model.Comment) {
 		return
 	}
 	type nameRow struct {
-		UID      string
+		UID      uint
 		Username string
 	}
 	var rows []nameRow
@@ -105,12 +105,12 @@ func (r *CommentRepo) fillReplyToNames(list []model.Comment) {
 		Where("id IN (SELECT reply_to_uid FROM comments WHERE root_id IN (?) AND parent_id IS NOT NULL)", r.rootIDsFromList(list)).
 		Find(&rows)
 
-	m := make(map[string]string)
+	m := make(map[uint]string)
 	for _, row := range rows {
 		m[row.UID] = row.Username
 	}
 	for i := range list {
-		if list[i].ReplyToUID != "" {
+		if list[i].ReplyToUID != 0 {
 			list[i].ReplyToName = m[list[i].ReplyToUID]
 		}
 	}