feat: implement save receipt functionality with transaction handling and DTOs

backend/prisma/migrations/20260509000000_remove_product_category_and_add_indexes/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable: Remove Product.category field (redundant with categoryId)
+ALTER TABLE `Product` DROP COLUMN `category`;
+
+-- AlterTable: Add index on ReceiptAlias.receiptName for faster lookups
+CREATE INDEX `ReceiptAlias_receiptName_idx` ON `ReceiptAlias`(`receiptName`);