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/pantry/dto/create-pantry-item.dto.js

@@ -19,4 +19,10 @@ __decorate([
     (0, class_validator_1.IsPositive)(),
     __metadata("design:type", Number)
 ], CreatePantryItemDto.prototype, "productId", void 0);
+__decorate([
+    (0, class_validator_1.IsOptional)(),
+    (0, class_validator_1.IsString)(),
+    (0, class_validator_1.MaxLength)(50),
+    __metadata("design:type", String)
+], CreatePantryItemDto.prototype, "location", void 0);
 //# sourceMappingURL=create-pantry-item.dto.js.map