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_mention_repo.go

@@ -1,8 +1,6 @@
 package repository
 
 import (
-	"strconv"
-
 	"metazone.cc/metalab/internal/model"
 	"metazone.cc/metalab/internal/service"
 )
@@ -21,7 +19,7 @@ func (r *CommentRepo) FindMentionsByComment(commentID uint) ([]model.CommentMent
 
 // SearchUsersByLevel 搜索指定等级及以上用户（用于@艾特），返回 uid/username/exp
 func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int) ([]struct {
-	UID      string
+	UID      uint
 	Username string
 	Exp      int
 }, error) {
@@ -42,30 +40,20 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 		Find(&list).Error
 
 	var res []struct {
-		UID      string
+		UID      uint
 		Username string
 		Exp      int
 	}
 	for _, u := range list {
 		res = append(res, struct {
-			UID      string
+			UID      uint
 			Username string
 			Exp      int
-		}{UID: strconv.FormatUint(uint64(u.UID), 10), Username: u.Username, Exp: u.Exp})
+		}{UID: u.UID, Username: u.Username, Exp: u.Exp})
 	}
 	return res, err
 }
 
-// GetUserUIDByID 通过 uint ID 获取 user 的 UID 字符串
-func (r *CommentRepo) GetUserUIDByID(userID uint) (string, error) {
-	var uid uint
-	err := r.db.Table("users").Select("id").Where("id = ?", userID).Scan(&uid).Error
-	if err != nil {
-		return "", err
-	}
-	return strconv.FormatUint(uint64(uid), 10), nil
-}
-
 // GetUsernameByID 通过 userID 获取用户名（用于通知内容）
 func (r *CommentRepo) GetUsernameByID(userID uint) (string, error) {
 	var username string