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/follow_repo.go

@@ -0,0 +1,112 @@
+package repository
+
+import (
+	"metazone.cc/metalab/internal/model"
+	"metazone.cc/metalab/internal/service"
+
+	"gorm.io/gorm"
+)
+
+// FollowRepo 关注数据访问
+type FollowRepo struct {
+	db *gorm.DB
+}
+
+// NewFollowRepo 构造函数
+func NewFollowRepo(db *gorm.DB) *FollowRepo {
+	return &FollowRepo{db: db}
+}
+
+// WithTx 基于事务连接创建 FollowRepo
+func (r *FollowRepo) WithTx(tx *gorm.DB) service.FollowStore {
+	return &FollowRepo{db: tx}
+}
+
+// Create 创建关注
+func (r *FollowRepo) Create(follow *model.UserFollow) error {
+	return r.db.Create(follow).Error
+}
+
+// Delete 取消关注
+func (r *FollowRepo) Delete(followerID, followeeID uint) error {
+	return r.db.Where("follower_id = ? AND followee_id = ?", followerID, followeeID).
+		Delete(&model.UserFollow{}).Error
+}
+
+// Exists 检查是否已关注
+func (r *FollowRepo) Exists(followerID, followeeID uint) (bool, error) {
+	var count int64
+	err := r.db.Model(&model.UserFollow{}).
+		Where("follower_id = ? AND followee_id = ?", followerID, followeeID).
+		Count(&count).Error
+	return count > 0, err
+}
+
+// CountFollowers 粉丝数
+func (r *FollowRepo) CountFollowers(userID uint) (int64, error) {
+	var count int64
+	err := r.db.Model(&model.UserFollow{}).Where("followee_id = ?", userID).Count(&count).Error
+	return count, err
+}
+
+// CountFollowing 关注数
+func (r *FollowRepo) CountFollowing(userID uint) (int64, error) {
+	var count int64
+	err := r.db.Model(&model.UserFollow{}).Where("follower_id = ?", userID).Count(&count).Error
+	return count, err
+}
+
+// ListFollowers 粉丝列表（分页，JOIN users 获取用户名）
+func (r *FollowRepo) ListFollowers(userID uint, offset, limit int) ([]model.UserFollow, int64, error) {
+	var total int64
+	if err := r.db.Model(&model.UserFollow{}).Where("followee_id = ?", userID).Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+	var items []model.UserFollow
+	err := r.db.Table("user_follows").
+		Select("user_follows.*, u.username AS follower_username").
+		Joins("INNER JOIN users u ON u.id = user_follows.follower_id").
+		Where("user_follows.followee_id = ?", userID).
+		Order("user_follows.created_at DESC").
+		Offset(offset).Limit(limit).
+		Scan(&items).Error
+	return items, total, err
+}
+
+// ListFollowing 关注列表（分页，JOIN users 获取用户名）
+func (r *FollowRepo) ListFollowing(userID uint, offset, limit int) ([]model.UserFollow, int64, error) {
+	var total int64
+	if err := r.db.Model(&model.UserFollow{}).Where("follower_id = ?", userID).Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+	var items []model.UserFollow
+	err := r.db.Table("user_follows").
+		Select("user_follows.*, u.username AS followee_username").
+		Joins("INNER JOIN users u ON u.id = user_follows.followee_id").
+		Where("user_follows.follower_id = ?", userID).
+		Order("user_follows.created_at DESC").
+		Offset(offset).Limit(limit).
+		Scan(&items).Error
+	return items, total, err
+}
+
+// IncrFollowersCount 原子增减粉丝数
+func (r *FollowRepo) IncrFollowersCount(userID uint, delta int) error {
+	return r.db.Model(&model.User{}).
+		Where("id = ?", userID).
+		UpdateColumn("followers_count", gorm.Expr("followers_count + ?", delta)).Error
+}
+
+// IncrFollowingCount 原子增减关注数
+func (r *FollowRepo) IncrFollowingCount(userID uint, delta int) error {
+	return r.db.Model(&model.User{}).
+		Where("id = ?", userID).
+		UpdateColumn("following_count", gorm.Expr("following_count + ?", delta)).Error
+}
+
+// GetFollowListPublic 查询用户关注列表公开性
+func (r *FollowRepo) GetFollowListPublic(userID uint) (bool, error) {
+	var user model.User
+	err := r.db.Select("follow_list_public").First(&user, userID).Error
+	return user.FollowListPublic, err
+}