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/router/deps_extra.go

@@ -74,6 +74,13 @@ func buildDeps(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSetting
 	reactionService := service.NewReactionService(db, reactionRepo)
 	reactionCtrl := controller.NewReactionController(reactionService)
 
+	// 关注系统
+	followRepo := repository.NewFollowRepo(db)
+	followService := service.NewFollowService(db, followRepo)
+	followService.SetNotifier(notificationService) // 关注通知
+	followCtrl := controller.NewFollowController(followService)
+	followPageCtrl := controller.NewFollowPageController(followService)
+
 	// 用户空间
 	spaceService := service.NewSpaceService(userRepo, postRepo)
 	spaceCtrl := controller.NewSpaceController(spaceService)
@@ -96,5 +103,7 @@ func buildDeps(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSetting
 		commentService:  commentService,
 		adminCommentCtrl: adminCommentCtrl,
 		reactionCtrl:    reactionCtrl,
+		followCtrl:      followCtrl,
+		followPageCtrl:  followPageCtrl,
 	}
 }