feat: @搜索优先展示已关注用户，结果限制最多5条

- SearchUsersByLevel SQL新增LEFT JOIN user_follows，按已关注优先+exp降序
- SearchUsers/followerUID参数贯穿repo→service→controller
- 继续输入时前端自动重新搜索新关键词

internal/repository/comment_mention_repo.go

@@ -67,7 +67,8 @@ func (r *CommentRepo) LoadMentionsForComments(comments []model.Comment) error {
 }
 
 // SearchUsersByLevel 搜索指定等级及以上用户（用于@艾特），返回 uid/username/avatar/exp
-func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int) ([]struct {
+// followerUID > 0 时按已关注优先排序，其次按exp降序
+func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel, limit int, followerUID uint) ([]struct {
 	UID      uint
 	Username string
 	Avatar   string
@@ -83,12 +84,17 @@ func (r *CommentRepo) SearchUsersByLevel(keyword string, minLevel int, limit int
 
 	threshold := model.LevelThresholds[minLevel]
 
-	err := r.db.Table("users").
-		Select("id AS uid, username, avatar, exp").
-		Where("username LIKE ? AND exp >= ? AND deleted_at IS NULL", "%"+keyword+"%", threshold).
-		Order("exp DESC").
-		Limit(limit).
-		Find(&list).Error
+	q := r.db.Table("users AS u").
+		Select("u.id AS uid, u.username, u.avatar, u.exp").
+		Where("u.username LIKE ? AND u.exp >= ? AND u.deleted_at IS NULL", "%"+keyword+"%", threshold).
+		Order("u.exp DESC")
+
+	if followerUID > 0 {
+		q = q.Joins("LEFT JOIN user_follows f ON f.followee_id = u.id AND f.follower_id = ?", followerUID).
+			Order("(f.follower_id IS NOT NULL) DESC, u.exp DESC")
+	}
+
+	err := q.Limit(limit).Find(&list).Error
 
 	var res []struct {
 		UID      uint