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/service/repository.go

@@ -164,5 +164,24 @@ type ReactionStore interface {
 	DeleteDislike(userID, postID uint) error
 	IncrPostLikes(postID uint, delta int) error
 	IncrPostDislikes(postID uint, delta int) error
+	GetPostAuthorID(postID uint) (uint, error)
+	UpsertDailyLikeSummary(authorUID uint, date string, likerUID string) error
+	GetUnnotifiedSummaries(userID uint) ([]model.DailyLikeSummary, error)
+	MarkSummaryNotified(id uint) error
 	WithTx(tx *gorm.DB) ReactionStore
 }
+
+// FollowStore FollowService 所需的最小仓储接口（ISP）
+type FollowStore interface {
+	Create(follow *model.UserFollow) error
+	Delete(followerID, followeeID uint) error
+	Exists(followerID, followeeID uint) (bool, error)
+	CountFollowers(userID uint) (int64, error)
+	CountFollowing(userID uint) (int64, error)
+	ListFollowers(userID uint, offset, limit int) ([]model.UserFollow, int64, error)
+	ListFollowing(userID uint, offset, limit int) ([]model.UserFollow, int64, error)
+	IncrFollowersCount(userID uint, delta int) error
+	IncrFollowingCount(userID uint, delta int) error
+	GetFollowListPublic(userID uint) (bool, error)
+	WithTx(tx *gorm.DB) FollowStore
+}