新功能(#76): 促销系统下单流程集成

- 新增促销规则/规格促销价/使用记录模型与幂等迁移
- EvaluatePromo 支持 new_user/inactive_user/campaign 规则判定
- purchase/preCreate 接入促销判定：命中促销时跳过百分比折扣
- activate 激活后按 order_no 幂等写入 promo_usage
- 订单模型和响应结构新增 promo_rule_id、promo_discount 字段

Co-authored-by: multica-agent <github@multica.ai>

internal/model/promo/promo.go

@@ -0,0 +1,58 @@
+package promo
+
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
+
+const (
+	RuleTypeNewUser      = "new_user"
+	RuleTypeInactiveUser = "inactive_user"
+	RuleTypeCampaign     = "campaign"
+)
+
+type Rule struct {
+	Id        int64          `gorm:"primaryKey"`
+	Name      string         `gorm:"type:varchar(100);not null;default:'';comment:Rule Name"`
+	Type      string         `gorm:"type:varchar(32);not null;default:'';comment:Rule Type"`
+	Params    string         `gorm:"type:json;not null;comment:Rule Params"`
+	Priority  int64          `gorm:"type:int;not null;default:0;comment:Priority"`
+	Enabled   bool           `gorm:"type:tinyint(1);not null;default:1;comment:Enabled"`
+	StartTime *time.Time     `gorm:"default:null;comment:Start Time"`
+	EndTime   *time.Time     `gorm:"default:null;comment:End Time"`
+	CreatedAt time.Time      `gorm:"<-:create;comment:Create Time"`
+	UpdatedAt time.Time      `gorm:"comment:Update Time"`
+	DeletedAt gorm.DeletedAt `gorm:"index;comment:Delete Time"`
+}
+
+func (Rule) TableName() string {
+	return "promo_rule"
+}
+
+type SubscribePromo struct {
+	Id          int64     `gorm:"primaryKey"`
+	SubscribeId int64     `gorm:"type:bigint unsigned;not null;comment:Subscribe ID"`
+	PromoRuleId int64     `gorm:"type:bigint unsigned;not null;comment:Promo Rule ID"`
+	PromoPrice  int64     `gorm:"type:bigint;not null;default:0;comment:Promo Price"`
+	CreatedAt   time.Time `gorm:"<-:create;comment:Create Time"`
+	UpdatedAt   time.Time `gorm:"comment:Update Time"`
+}
+
+func (SubscribePromo) TableName() string {
+	return "subscribe_promo"
+}
+
+type Usage struct {
+	Id          int64     `gorm:"primaryKey"`
+	UserId      int64     `gorm:"type:bigint unsigned;not null;comment:User ID"`
+	PromoRuleId int64     `gorm:"type:bigint unsigned;not null;comment:Promo Rule ID"`
+	SubscribeId int64     `gorm:"type:bigint unsigned;not null;comment:Subscribe ID"`
+	OrderNo     string    `gorm:"type:varchar(255);not null;default:'';comment:Order No"`
+	PromoPrice  int64     `gorm:"type:bigint;not null;default:0;comment:Promo Price"`
+	CreatedAt   time.Time `gorm:"<-:create;comment:Create Time"`
+}
+
+func (Usage) TableName() string {
+	return "promo_usage"
+}