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/controller/comment_action_controller.go

@@ -41,7 +41,7 @@ func (ctrl *CommentController) Delete(c *gin.Context) {
 
 // SearchUsers GET /api/users/search?q=keyword
 func (ctrl *CommentController) SearchUsers(c *gin.Context) {
-	_, _, ok := common.GetGinUser(c)
+	uid, _, ok := common.GetGinUser(c)
 	if !ok {
 		common.Error(c, http.StatusUnauthorized, "请先登录")
 		return
@@ -53,7 +53,7 @@ func (ctrl *CommentController) SearchUsers(c *gin.Context) {
 		return
 	}
 
-	users, err := ctrl.commentService.SearchUsers(q)
+	users, err := ctrl.commentService.SearchUsers(q, uid)
 	if err != nil {
 		common.Error(c, http.StatusInternalServerError, "搜索用户失败")
 		return

internal/controller/interfaces.go

@@ -115,7 +115,7 @@ type commentUseCase interface {
 	Delete(commentID uint, requesterID uint, isAdmin bool) error
 	ListRootComments(postID uint, page, pageSize int) ([]model.Comment, int64, error)
 	ListReplies(rootID uint) ([]model.Comment, error)
-	SearchUsers(keyword string) ([]model.UserSearchResult, error)
+	SearchUsers(keyword string, followerUID uint) ([]model.UserSearchResult, error)
 }
 
 // reactionUseCase ReactionController 对 ReactionService 的最小依赖（ISP：4 个方法）