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/dist/receipt-import/dto/create-unit-mapping.dto.js

@@ -0,0 +1,31 @@
+"use strict";
+var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
+    var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
+    if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
+    else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
+    return c > 3 && r && Object.defineProperty(target, key, r), r;
+};
+var __metadata = (this && this.__metadata) || function (k, v) {
+    if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
+};
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.CreateUnitMappingDto = void 0;
+const class_validator_1 = require("class-validator");
+class CreateUnitMappingDto {
+}
+exports.CreateUnitMappingDto = CreateUnitMappingDto;
+__decorate([
+    (0, class_validator_1.IsInt)(),
+    __metadata("design:type", Number)
+], CreateUnitMappingDto.prototype, "productId", void 0);
+__decorate([
+    (0, class_validator_1.IsString)(),
+    (0, class_validator_1.MinLength)(1),
+    __metadata("design:type", String)
+], CreateUnitMappingDto.prototype, "originalUnit", void 0);
+__decorate([
+    (0, class_validator_1.IsString)(),
+    (0, class_validator_1.MinLength)(1),
+    __metadata("design:type", String)
+], CreateUnitMappingDto.prototype, "preferredUnit", void 0);
+//# sourceMappingURL=create-unit-mapping.dto.js.map