feat(distribution): 独立分销后台账号体系、多租户隔离与主站后台分销管理GUI

- 独立账号鉴权：分销后台支持独立登录页面、Bearer+Cookie 双模鉴权与租户资料接口
- 多租户隔离：分销商成员强制锁定自身站点数据范围，防止横向越权；平台超管享有全局视界
- 一键开通分销商：平台端支持一键录入站点、管理员账号、分配角色并原子化初始化佣金规则
- 主后台分销分组：在主站后台增加「分销管理」导航分组，提供站点管理、规则配置、订单监控与结算管理 GUI
- 门禁与联调：补充完整单测，全量代码门禁 check.sh 100% 通过，dev.sh 支持前后端与分销后台一键同启

backend/internal/modules/distribution/repository_test.go

@@ -16,7 +16,7 @@ func distributionTestDB(t *testing.T) *gorm.DB {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if err := db.AutoMigrate(&model.ChatConversation{}, &model.RentalOrder{}, &model.AuditLog{}, &model.PaymentMerchantConfig{}, &model.DistributionSite{}, &model.DistributionSiteMember{}, &model.DistributionRuleVersion{}, &model.DistributionOrderFinancial{}, &model.DistributionOrderParty{}, &model.DistributionSiteAccount{}, &model.DistributionSitePaymentChannel{}, &model.DistributionSettlementBatch{}, &model.DistributionSettlementItem{}); err != nil {
+	if err := db.AutoMigrate(&model.AdminUser{}, &model.Role{}, &model.ChatConversation{}, &model.RentalOrder{}, &model.AuditLog{}, &model.PaymentMerchantConfig{}, &model.DistributionSite{}, &model.DistributionSiteMember{}, &model.DistributionRuleVersion{}, &model.DistributionOrderFinancial{}, &model.DistributionOrderParty{}, &model.DistributionSiteAccount{}, &model.DistributionSitePaymentChannel{}, &model.DistributionSettlementBatch{}, &model.DistributionSettlementItem{}); err != nil {
 		t.Fatal(err)
 	}
 	return db