fix: 修复代理卡密列表数据不显示问题

- Card模型添加AgentID字段
- 生成卡密时设置AgentID
- handleGetCards返回正确格式的数据(包含app_name, card_type_name, code等)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/model/models.go

@@ -286,6 +286,7 @@ type Card struct {
 	CardTypeID    uint           `gorm:"index;not null" json:"card_type_id"`
 	CardKey       string         `gorm:"uniqueIndex;size:100" json:"card_key"`
 	CreatorID     uint           `gorm:"index" json:"creator_id"`
+	AgentID       *uint          `gorm:"index" json:"agent_id"`
 	AppUserID     *uint          `gorm:"index" json:"app_user_id"`
 	Status        string         `gorm:"size:20;default:unused;index" json:"status"`
 	UsedAt        *time.Time     `json:"used_at"`
@@ -297,6 +298,7 @@ type Card struct {
 	Application *Application `gorm:"foreignKey:ApplicationID" json:"application,omitempty"`
 	CardType    CardType     `gorm:"foreignKey:CardTypeID" json:"card_type,omitempty"`
 	Creator     User         `gorm:"foreignKey:CreatorID" json:"creator,omitempty"`
+	Agent       *User        `gorm:"foreignKey:AgentID" json:"agent,omitempty"`
 	AppUser     *AppUser     `gorm:"foreignKey:AppUserID" json:"app_user,omitempty"`
 }