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;

backend/prisma/schema.prisma

@@ -158,7 +158,8 @@ model Address {
   city        String
   country     String      @default("Deutschland")
   isDefault   Boolean     @default(false)
-  contracts   Contract[]
+  contractsAsDelivery   Contract[] @relation("DeliveryAddress")
+  contractsAsBilling    Contract[] @relation("BillingAddress")
   createdAt   DateTime    @default(now())
   updatedAt   DateTime    @updatedAt
 }
@@ -481,8 +482,13 @@ model Contract {
   contractCategoryId      Int?
   contractCategory        ContractCategory? @relation(fields: [contractCategoryId], references: [id])
 
+  // Lieferadresse
   addressId               Int?
-  address                 Address?        @relation(fields: [addressId], references: [id])
+  address                 Address?        @relation("DeliveryAddress", fields: [addressId], references: [id])
+
+  // Rechnungsadresse (falls leer, wird Lieferadresse verwendet)
+  billingAddressId        Int?
+  billingAddress          Address?        @relation("BillingAddress", fields: [billingAddressId], references: [id])
 
   bankCardId              Int?
   bankCard                BankCard?       @relation(fields: [bankCardId], references: [id])