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/model.go

@@ -0,0 +1,48 @@
+package promo
+
+import (
+	"context"
+
+	"github.com/redis/go-redis/v9"
+	"gorm.io/gorm"
+)
+
+type RuleWithPrice struct {
+	Rule
+	PromoPrice int64 `gorm:"column:promo_price"`
+}
+
+type Model interface {
+	QueryEligibleRules(ctx context.Context, subscribeId int64) ([]*RuleWithPrice, error)
+	InsertUsage(ctx context.Context, data *Usage, tx ...*gorm.DB) error
+}
+
+type defaultPromoModel struct {
+	db *gorm.DB
+}
+
+func NewModel(db *gorm.DB, _ *redis.Client) Model {
+	return &defaultPromoModel{db: db}
+}
+
+func (m *defaultPromoModel) QueryEligibleRules(ctx context.Context, subscribeId int64) ([]*RuleWithPrice, error) {
+	var list []*RuleWithPrice
+	err := m.db.WithContext(ctx).
+		Table("promo_rule AS pr").
+		Select("pr.*, sp.promo_price").
+		Joins("JOIN subscribe_promo AS sp ON sp.promo_rule_id = pr.id").
+		Where("sp.subscribe_id = ? AND sp.promo_price > 0 AND pr.enabled = ?", subscribeId, true).
+		Where("pr.deleted_at IS NULL").
+		Order("pr.priority DESC").
+		Order("pr.id ASC").
+		Find(&list).Error
+	return list, err
+}
+
+func (m *defaultPromoModel) InsertUsage(ctx context.Context, data *Usage, tx ...*gorm.DB) error {
+	db := m.db.WithContext(ctx)
+	if len(tx) > 0 {
+		db = tx[0].WithContext(ctx)
+	}
+	return db.Model(&Usage{}).Create(data).Error
+}