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/receipt-import.controller.d.ts

@@ -1,5 +1,6 @@
 import { ReceiptImportService } from './receipt-import.service';
 import { ParsedReceiptItem } from './dto/parsed-receipt-item.dto';
+import { CreateUnitMappingDto } from './dto/create-unit-mapping.dto';
 export declare class ReceiptImportController {
     private readonly receiptImportService;
     constructor(receiptImportService: ReceiptImportService);
@@ -7,4 +8,5 @@ export declare class ReceiptImportController {
     refreshCategories(): Promise<{
         message: string;
     }>;
+    upsertUnitMapping(dto: CreateUnitMappingDto, req?: any): Promise<any>;
 }