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/service/repository.go

@@ -155,7 +155,7 @@ type commentStore interface {
 	CreateMention(mention *model.CommentMention) error
 	FindPostIDByComment(commentID uint) (uint, error)
 	FindPostAuthorID(postID uint) (uint, error)
-	SearchUsersByLevel(keyword string, minLevel, limit int) ([]struct {
+	SearchUsersByLevel(keyword string, minLevel, limit int, followerUID uint) ([]struct {
 		UID      uint
 		Username string
 		Avatar   string