feat: 客户关联产品并支持按产品圈选通知群体

新增客户-产品多对多关联，列表可按产品筛选并复制邮箱；发证/下单自动回填历史关联。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/model/customer.go

@@ -17,6 +17,7 @@ type Customer struct {
 	Notes     string         `json:"notes" gorm:"type:text"`
 	Status    int            `json:"status" gorm:"default:1"` // 1: 启用, 0: 禁用
 	Licenses  []License      `json:"licenses,omitempty" gorm:"foreignKey:CustomerID"`
+	Products  []Product      `json:"products,omitempty" gorm:"many2many:customer_products;"`
 	CreatedAt time.Time      `json:"created_at"`
 	UpdatedAt time.Time      `json:"updated_at"`
 	DeletedAt gorm.DeletedAt `json:"deleted_at,omitempty" gorm:"index"`