fix(distribution): 修正商品归属回填、禁止全站灌货并拒绝无效站点头

用货架写入时间与上传来源回滚误标产地，不再依赖绝对时间戳；同步仅收本站上传与显式勾选。

backend/internal/modules/listing/service.go

@@ -32,6 +32,7 @@ var (
 	ErrExternalListingRequired  = errors.New("external listing required")
 	ErrExternalAdjustment       = errors.New("invalid external listing adjustment")
 	ErrSaleRatioOutOfRange      = errors.New("sale ratio out of range")
+	ErrDistributionSiteInvalid  = errors.New("invalid distribution site")
 )
 
 type Service struct {