feat: add unit mapping functionality

- Added new API path for unit mappings in `api_paths.dart`.
- Implemented `upsertUnitMapping` method in `ImportRepository` to handle unit mapping creation.
- Updated `ReceiptImportTab` to learn and save unit mappings during receipt import.
- Created DTO for unit mapping with validation in `create-unit-mapping.dto.ts`.
- Added SQL migration for `UnitMapping` table creation with necessary constraints.

backend/prisma/migrations/20260507153000_add_unit_mapping/migration.sql

@@ -0,0 +1,22 @@
+CREATE TABLE `UnitMapping` (
+  `id` INTEGER NOT NULL AUTO_INCREMENT,
+  `productId` INTEGER NOT NULL,
+  `originalUnit` VARCHAR(191) NOT NULL,
+  `preferredUnit` VARCHAR(191) NOT NULL,
+  `userId` INTEGER NOT NULL,
+
+  UNIQUE INDEX `UnitMapping_productId_originalUnit_userId_key`(`productId`, `originalUnit`, `userId`),
+  INDEX `UnitMapping_productId_idx`(`productId`),
+  INDEX `UnitMapping_userId_idx`(`userId`),
+  PRIMARY KEY (`id`)
+) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+
+ALTER TABLE `UnitMapping`
+  ADD CONSTRAINT `UnitMapping_productId_fkey`
+  FOREIGN KEY (`productId`) REFERENCES `Product`(`id`)
+  ON DELETE CASCADE ON UPDATE CASCADE;
+
+ALTER TABLE `UnitMapping`
+  ADD CONSTRAINT `UnitMapping_userId_fkey`
+  FOREIGN KEY (`userId`) REFERENCES `User`(`id`)
+  ON DELETE CASCADE ON UPDATE CASCADE;