feat: 用户列表显示订阅分组及剩余天数

- User模型新增Subscriptions关联
- 用户列表批量加载订阅信息避免N+1查询
- GroupBadge组件支持显示剩余天数（过期红色、<=3天红色、<=7天橙色）
- 用户管理页面新增订阅分组列

backend/internal/model/user.go

@@ -22,7 +22,8 @@ type User struct {
 	DeletedAt     gorm.DeletedAt `gorm:"index" json:"-"`
 
 	// 关联
-	ApiKeys []ApiKey `gorm:"foreignKey:UserID" json:"api_keys,omitempty"`
+	ApiKeys       []ApiKey           `gorm:"foreignKey:UserID" json:"api_keys,omitempty"`
+	Subscriptions []UserSubscription `gorm:"foreignKey:UserID" json:"subscriptions,omitempty"`
 }
 
 func (User) TableName() string {

backend/internal/repository/user_repo.go

@@ -73,10 +73,37 @@ func (r *UserRepository) ListWithFilters(ctx context.Context, params pagination.
 		return nil, nil, err
 	}
 
+	// Query users with pagination (reuse the same db with filters applied)
 	if err := db.Offset(params.Offset()).Limit(params.Limit()).Order("id DESC").Find(&users).Error; err != nil {
 		return nil, nil, err
 	}
 
+	// Batch load subscriptions for all users (avoid N+1)
+	if len(users) > 0 {
+		userIDs := make([]int64, len(users))
+		userMap := make(map[int64]*model.User, len(users))
+		for i := range users {
+			userIDs[i] = users[i].ID
+			userMap[users[i].ID] = &users[i]
+		}
+
+		// Query active subscriptions with groups in one query
+		var subscriptions []model.UserSubscription
+		if err := r.db.WithContext(ctx).
+			Preload("Group").
+			Where("user_id IN ? AND status = ?", userIDs, model.SubscriptionStatusActive).
+			Find(&subscriptions).Error; err != nil {
+			return nil, nil, err
+		}
+
+		// Associate subscriptions with users
+		for i := range subscriptions {
+			if user, ok := userMap[subscriptions[i].UserID]; ok {
+				user.Subscriptions = append(user.Subscriptions, subscriptions[i])
+			}
+		}
+	}
+
 	pages := int(total) / params.Limit()
 	if int(total)%params.Limit() > 0 {
 		pages++