fix(invitation): 返利归因统一为「订单所属站点」，买卖两端一致

- 买家端不再强制记主站（原 buyerSiteID=0），卖家端不再记拆分方站点，两端统一取 orderSiteID
- orderSiteID = 订单所属站点（>0 时）；订单未记站点但存在分销拆分时回退到拆分方站点，避免主站替分站订单买单
- 该站点同时用于取返利配置(GetRebateConfig)、返利流水与钱包流水的 distribution_site_id，分站后台由此能看到本站返利支出
- 基数口径未改动，非分销订单行为与改前一致；历史 9 笔数据不订正，规则自此生效
- 测试：更新跨站用例断言；新增订单站点归因用例（校验站点专属比例 0.5 与钱包流水站点维度）

backend/internal/modules/invitation/repository.go

@@ -386,27 +386,30 @@ func (r *Repository) ProcessOrderInvitationRebate(ctx context.Context, tx *gorm.
 
 	now := time.Now()
 
+	// 返利归因站点（统一规则）：成本一律由「订单所属站点」承担，并使用该站点自己的返利配置。
+	// 订单未记录站点但存在分销拆分时，回退到拆分方站点（钱实际归它），
+	// 避免再次出现「主站替分站订单买单」的倒贴。
+	orderSiteID := distributorSiteID
+	if order.DistributionSiteID != nil && *order.DistributionSiteID > 0 {
+		orderSiteID = *order.DistributionSiteID
+	}
+
 	// 1. 买家端处理（租客 RenterID）
 	var buyerInvitation model.UserInvitation
 	if err := tx.Where("invitee_id = ? AND status = ?", order.RenterID, StatusActive).First(&buyerInvitation).Error; err == nil {
 		if buyerInvitation.VerifiedAt != nil {
 			var inviter model.User
 			if err := tx.First(&inviter, buyerInvitation.InviterID).Error; err == nil && inviter.RealnameStatus == "verified" && inviter.ID != order.RenterID {
-				// 买家端基数：若有分销售卖拆分按 platformSellCent，否则按整单毛利对半拆
+				// 买家端基数：有分销售卖拆分按 platformSellCent，否则按整单毛利对半拆；归因站点统一取 orderSiteID
 				var buyerBasisCent int64
-				var buyerSiteID uint64
 				if platformSellCent > 0 {
 					buyerBasisCent = int64(math.Floor(float64(platformSellCent) / 2.0))
-					buyerSiteID = 0
 				} else {
 					buyerBasisCent = int64(math.Floor(float64(orderGrossProfitCent) / 2.0))
-					if order.DistributionSiteID != nil {
-						buyerSiteID = *order.DistributionSiteID
-					}
 				}
 
 				if buyerBasisCent > 0 {
-					cfg, _ := r.GetRebateConfig(ctx, buyerSiteID)
+					cfg, _ := r.GetRebateConfig(ctx, orderSiteID)
 					if cfg == nil || cfg.Enabled {
 						rate, cycle, days := CalculateRebateRate(*buyerInvitation.VerifiedAt, now, cfg)
 						rebateCent := int64(math.Round(float64(buyerBasisCent) * rate))
@@ -415,7 +418,7 @@ func (r *Repository) ProcessOrderInvitationRebate(ctx context.Context, tx *gorm.
 							ledger := model.InvitationRebateLedger{
 								OrderID:              order.ID,
 								OrderNo:              order.OrderNo,
-								DistributionSiteID:   buyerSiteID,
+								DistributionSiteID:   orderSiteID,
 								InviterID:            buyerInvitation.InviterID,
 								InviteeID:            order.RenterID,
 								TradeRole:            "buyer",
@@ -432,7 +435,7 @@ func (r *Repository) ProcessOrderInvitationRebate(ctx context.Context, tx *gorm.
 							if err := tx.Clauses(clause.OnConflict{DoNothing: true}).Create(&ledger).Error; err == nil && ledger.ID > 0 {
 								_ = wallet.AppendEntries(tx, wallet.Entry{
 									UserID:             buyerInvitation.InviterID,
-									DistributionSiteID: buyerSiteID,
+									DistributionSiteID: orderSiteID,
 									Direction:          "in",
 									AmountCent:         rebateCent,
 									BalanceType:        "available",
@@ -454,21 +457,16 @@ func (r *Repository) ProcessOrderInvitationRebate(ctx context.Context, tx *gorm.
 		if sellerInvitation.VerifiedAt != nil {
 			var inviter model.User
 			if err := tx.First(&inviter, sellerInvitation.InviterID).Error; err == nil && inviter.RealnameStatus == "verified" && inviter.ID != order.OwnerID {
-				// 卖家端基数：若有分站收号拆分按 distributorReceiveCent（从分站出），否则按整单毛利对半拆
+				// 卖家端基数：有分站收号拆分按 distributorReceiveCent，否则按整单毛利对半拆；归因站点统一取 orderSiteID
 				var sellerBasisCent int64
-				var sellerSiteID uint64
 				if distributorReceiveCent > 0 {
 					sellerBasisCent = int64(math.Floor(float64(distributorReceiveCent) / 2.0))
-					sellerSiteID = distributorSiteID
 				} else {
 					sellerBasisCent = int64(math.Floor(float64(orderGrossProfitCent) / 2.0))
-					if order.DistributionSiteID != nil {
-						sellerSiteID = *order.DistributionSiteID
-					}
 				}
 
 				if sellerBasisCent > 0 {
-					cfg, _ := r.GetRebateConfig(ctx, sellerSiteID)
+					cfg, _ := r.GetRebateConfig(ctx, orderSiteID)
 					if cfg == nil || cfg.Enabled {
 						rate, cycle, days := CalculateRebateRate(*sellerInvitation.VerifiedAt, now, cfg)
 						rebateCent := int64(math.Round(float64(sellerBasisCent) * rate))
@@ -477,7 +475,7 @@ func (r *Repository) ProcessOrderInvitationRebate(ctx context.Context, tx *gorm.
 							ledger := model.InvitationRebateLedger{
 								OrderID:              order.ID,
 								OrderNo:              order.OrderNo,
-								DistributionSiteID:   sellerSiteID,
+								DistributionSiteID:   orderSiteID,
 								InviterID:            sellerInvitation.InviterID,
 								InviteeID:            order.OwnerID,
 								TradeRole:            "seller",
@@ -494,7 +492,7 @@ func (r *Repository) ProcessOrderInvitationRebate(ctx context.Context, tx *gorm.
 							if err := tx.Clauses(clause.OnConflict{DoNothing: true}).Create(&ledger).Error; err == nil && ledger.ID > 0 {
 								_ = wallet.AppendEntries(tx, wallet.Entry{
 									UserID:             sellerInvitation.InviterID,
-									DistributionSiteID: sellerSiteID,
+									DistributionSiteID: orderSiteID,
 									Direction:          "in",
 									AmountCent:         rebateCent,
 									BalanceType:        "available",