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/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)
 }