feat(换货): AUG26-005 换货业务数据迁移状态与失败恢复

- 新增成对迁移 000222：tb_exchange_order 增加非空 migration_status 与
  migration_failure_reason，按既有 migrate_data/migration_completed 回填历史，
  并加四值 CHECK 约束，不新增索引
- 模型与常量定义四种迁移状态及中文名称，保留既有布尔字段兼容语义
- 物流换货创建恒 not_migrated，发货按请求落 pending/not_migrated，
  完成成功写 migrated/not_migrated 并清空失败原因、同步兼容字段
- 直接换货创建即完成，任一步失败整体回滚，不持久化换货单、不产生 failed
- 迁移失败回滚全部业务修改后，在独立短事务内条件更新 failed 与安全失败原因
  并写失败审计，RowsAffected 为 0 时跳过状态写入但仍写审计
- failed 物流单重试仅限超级管理员或平台用户，授权以锁内 FOR UPDATE 判定为准，
  重试从钱包余额起整表重跑；非 failed 单沿用既有完成门禁
- 列表与详情返回迁移状态与中文名称，仅 failed 返回失败原因；既有三字段保持兼容
- 换货导出在「状态」列后新增中文「迁移状态」列，不导出失败原因
- 同步 order-refund-exchange 主 spec 与验证证据，归档本 Change
- 登记 KNOWN-ISSUE-001：既有标签复制 OnConflict 未声明部分索引谓词（42P10），
  旧资产带标签时迁移最后一步失败，待另立变更修复

internal/query/exchange/list.go

@@ -126,6 +126,11 @@ func projectExchangeOrder(order *model.ExchangeOrder) *dto.ExchangeOrderResponse
 	if order.DeletedAt.Valid {
 		deletedAt = &order.DeletedAt.Time
 	}
+	// 失败原因只在迁移失败时对外可见，其他状态一律不返回，避免把历史原因误读为当前状态。
+	failureReason := ""
+	if order.MigrationStatus == constants.ExchangeMigrationStatusFailed {
+		failureReason = order.MigrationFailureReason
+	}
 	return &dto.ExchangeOrderResponse{
 		ID: order.ID, ExchangeNo: order.ExchangeNo,
 		FlowType: effectiveFlowType(order.FlowType), FlowTypeName: constants.GetExchangeFlowTypeName(order.FlowType),
@@ -134,7 +139,9 @@ func projectExchangeOrder(order *model.ExchangeOrder) *dto.ExchangeOrderResponse
 		RecipientName: order.RecipientName, RecipientPhone: order.RecipientPhone, RecipientAddress: order.RecipientAddress,
 		ExpressCompany: order.ExpressCompany, ExpressNo: order.ExpressNo,
 		MigrateData: order.MigrateData, MigrationCompleted: order.MigrationCompleted, MigrationBalance: order.MigrationBalance,
-		ShippedAt: order.ShippedAt, CompletedAt: order.CompletedAt,
+		MigrationStatus: order.MigrationStatus, MigrationStatusName: constants.GetExchangeMigrationStatusName(order.MigrationStatus),
+		MigrationFailureReason: failureReason,
+		ShippedAt:              order.ShippedAt, CompletedAt: order.CompletedAt,
 		ExchangeReason: order.ExchangeReason, Remark: order.Remark,
 		Status: order.Status, StatusName: constants.GetExchangeStatusName(order.Status), StatusText: constants.GetExchangeStatusName(order.Status),
 		ShopID: order.ShopID, CreatedAt: order.CreatedAt, UpdatedAt: order.UpdatedAt, DeletedAt: deletedAt,