seperate delivery and billig adresses in contract added

backend/prisma/migrations/20260204074019_add_billing_address/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE `Contract` ADD COLUMN `billingAddressId` INTEGER NULL;
+
+-- AddForeignKey
+ALTER TABLE `Contract` ADD CONSTRAINT `Contract_billingAddressId_fkey` FOREIGN KEY (`billingAddressId`) REFERENCES `Address`(`id`) ON DELETE SET NULL ON UPDATE CASCADE;