feat: 实现关注系统 + 通知侧边栏分类 TAB + 点赞聚合通知

- 新增关注系统：UserFollow 模型、FollowService（toggle/status/列表隐私控制）
- User 新增 FollowersCount/FollowingCount/FollowListPublic/NotifyPrefs 字段
- Space 页面增加四态关注按钮（关注/已关注/回关/已互粉）+ 粉丝/关注数链接
- 新增 /space/:uid/followers 和 /space/:uid/following 列表页
- 新增 NotifyFollow/NotifyLikeAggregated 通知类型
- ReactionService 点赞时写入 daily_like_summary，访问时生成聚合通知
- FollowService 关注时触发 NotifyFollow 通知
- 消息中心侧边栏升级为分类 TAB（全部/系统通知/@艾特/点赞/关注）
- NotificationService 新增 ListByCategory 按分类分页查询

internal/repository/reaction_repo.go

@@ -75,3 +75,47 @@ func (r *ReactionRepo) IncrPostDislikes(postID uint, delta int) error {
 		Where("id = ?", postID).
 		UpdateColumn("dislikes_count", gorm.Expr("dislikes_count + ?", delta)).Error
 }
+
+// GetPostAuthorID 查询文章作者ID
+func (r *ReactionRepo) GetPostAuthorID(postID uint) (uint, error) {
+	var post model.Post
+	err := r.db.Select("user_id").First(&post, postID).Error
+	return post.UserID, err
+}
+
+// UpsertDailyLikeSummary 追加或创建每日点赞汇总
+func (r *ReactionRepo) UpsertDailyLikeSummary(authorUID uint, date string, likerUID string) error {
+	var existing model.DailyLikeSummary
+	err := r.db.Where("author_uid = ? AND date = ?", authorUID, date).First(&existing).Error
+	if err == gorm.ErrRecordNotFound {
+		return r.db.Create(&model.DailyLikeSummary{
+			AuthorUID: authorUID,
+			Date:      date,
+			LikerUIDs: likerUID,
+			Notified:  false,
+		}).Error
+	}
+	if err != nil {
+		return err
+	}
+	// 追加 likerUID
+	newUids := existing.LikerUIDs
+	if newUids == "" {
+		newUids = likerUID
+	} else {
+		newUids = newUids + "," + likerUID
+	}
+	return r.db.Model(&existing).Update("liker_uids", newUids).Error
+}
+
+// GetUnnotifiedSummaries 查询未通知的每日点赞汇总
+func (r *ReactionRepo) GetUnnotifiedSummaries(userID uint) ([]model.DailyLikeSummary, error) {
+	var summaries []model.DailyLikeSummary
+	err := r.db.Where("author_uid = ? AND notified = ?", userID, false).Find(&summaries).Error
+	return summaries, err
+}
+
+// MarkSummaryNotified 标记汇总已通知
+func (r *ReactionRepo) MarkSummaryNotified(id uint) error {
+	return r.db.Model(&model.DailyLikeSummary{}).Where("id = ?", id).Update("notified", true).Error
+}