新功能(#77): 套餐列表 API 返回规格级促销信息

将 promo 字段从 Subscribe 顶层移至 SubscribeDiscount（discount[] 项内），
促销查询按 subscribe_id + quantity 维度写入，每个规格独立命中最高优先级规则。

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

internal/logic/public/subscribe/promo.go

@@ -25,6 +25,7 @@ const (
 
 type subscribePromoCandidate struct {
 	SubscribeId int64      `gorm:"column:subscribe_id"`
+	Quantity    int64      `gorm:"column:quantity"`
 	RuleName    string     `gorm:"column:rule_name"`
 	RuleType    string     `gorm:"column:rule_type"`
 	PromoPrice  int64      `gorm:"column:promo_price"`
@@ -38,8 +39,8 @@ type promoRuleParams struct {
 	InactiveMonths int   `json:"inactive_months"`
 }
 
-func loadSubscribePromoMap(ctx context.Context, svcCtx *svc.ServiceContext, subscribeIDs []int64) (map[int64]*types.SubscribePromo, error) {
-	result := make(map[int64]*types.SubscribePromo)
+func loadSubscribePromoMap(ctx context.Context, svcCtx *svc.ServiceContext, subscribeIDs []int64) (map[int64]map[int64]*types.SubscribePromo, error) {
+	result := make(map[int64]map[int64]*types.SubscribePromo)
 	if len(subscribeIDs) == 0 || svcCtx == nil || svcCtx.DB == nil {
 		return result, nil
 	}
@@ -56,7 +57,7 @@ func loadSubscribePromoMap(ctx context.Context, svcCtx *svc.ServiceContext, subs
 	evaluator := promoEligibilityEvaluator{ctx: ctx, db: svcCtx.DB, userInfo: userInfo}
 	now := time.Now()
 	for _, candidate := range candidates {
-		if _, exists := result[candidate.SubscribeId]; exists {
+		if _, exists := result[candidate.SubscribeId][candidate.Quantity]; exists {
 			continue
 		}
 		if !candidate.isActive(now) {
@@ -69,7 +70,10 @@ func loadSubscribePromoMap(ctx context.Context, svcCtx *svc.ServiceContext, subs
 		if !ok {
 			continue
 		}
-		result[candidate.SubscribeId] = &types.SubscribePromo{
+		if _, exists := result[candidate.SubscribeId]; !exists {
+			result[candidate.SubscribeId] = make(map[int64]*types.SubscribePromo)
+		}
+		result[candidate.SubscribeId][candidate.Quantity] = &types.SubscribePromo{
 			RuleName:   candidate.RuleName,
 			RuleType:   candidate.RuleType,
 			PromoPrice: candidate.PromoPrice,
@@ -84,7 +88,7 @@ func querySubscribePromoCandidates(ctx context.Context, svcCtx *svc.ServiceConte
 	var candidates []subscribePromoCandidate
 	query := svcCtx.DB.WithContext(ctx).
 		Table("subscribe_promo AS sp").
-		Select("sp.subscribe_id, sp.promo_price, pr.name AS rule_name, pr.type AS rule_type, pr.params, pr.start_time, pr.end_time").
+		Select("sp.subscribe_id, sp.quantity, sp.promo_price, pr.name AS rule_name, pr.type AS rule_type, pr.params, pr.start_time, pr.end_time").
 		Joins("JOIN promo_rule AS pr ON pr.id = sp.promo_rule_id AND pr.deleted_at IS NULL").
 		Where("sp.subscribe_id IN ? AND sp.promo_price > 0 AND pr.enabled = ?", subscribeIDs, true)
 	if !loggedIn {
@@ -92,6 +96,7 @@ func querySubscribePromoCandidates(ctx context.Context, svcCtx *svc.ServiceConte
 	}
 	err := query.
 		Order("sp.subscribe_id ASC").
+		Order("sp.quantity ASC").
 		Order("pr.priority DESC").
 		Order("pr.id ASC").
 		Scan(&candidates).Error