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/notification_repo.go

@@ -60,3 +60,48 @@ func (r *NotificationRepo) MarkAllRead(userID uint) error {
 		Where("user_id = ? AND is_read = false", userID).
 		Update("is_read", true).Error
 }
+
+// ListByUserAndType 按类型分页查询用户消息
+func (r *NotificationRepo) ListByUserAndType(userID uint, notifyType string, offset, limit int) ([]model.Notification, error) {
+	var list []model.Notification
+	err := r.db.Where("user_id = ? AND notify_type = ?", userID, notifyType).
+		Order("created_at DESC").
+		Offset(offset).Limit(limit).
+		Find(&list).Error
+	return list, err
+}
+
+// CountByUserAndType 按类型统计用户消息数
+func (r *NotificationRepo) CountByUserAndType(userID uint, notifyType string) (int64, error) {
+	var count int64
+	err := r.db.Model(&model.Notification{}).Where("user_id = ? AND notify_type = ?", userID, notifyType).Count(&count).Error
+	return count, err
+}
+
+// CountUnreadByType 按类型统计用户未读消息数
+func (r *NotificationRepo) CountUnreadByType(userID uint, notifyType string) (int64, error) {
+	var count int64
+	err := r.db.Model(&model.Notification{}).
+		Where("user_id = ? AND is_read = false AND notify_type = ?", userID, notifyType).
+		Count(&count).Error
+	return count, err
+}
+
+// GetUnnotifiedDailyLikeSummaries 获取未通知的每日点赞汇总
+func (r *NotificationRepo) GetUnnotifiedDailyLikeSummaries(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
+}
+
+// MarkDailyLikeSummaryNotified 标记汇总已通知
+func (r *NotificationRepo) MarkDailyLikeSummaryNotified(id uint) error {
+	return r.db.Model(&model.DailyLikeSummary{}).Where("id = ?", id).Update("notified", true).Error
+}
+
+// GetUsernameByID 根据用户ID获取用户名
+func (r *NotificationRepo) GetUsernameByID(userID uint) (string, error) {
+	var user model.User
+	err := r.db.Select("username").First(&user, userID).Error
+	return user.Username, err
+}