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/pantry.service.d.ts

@@ -24,6 +24,7 @@ export declare class PantryService {
         createdAt: Date;
         updatedAt: Date;
         productId: number;
+        location: string | null;
         userId: number;
     })[]>;
     create(userId: number, data: CreatePantryItemDto): Promise<{
@@ -47,6 +48,7 @@ export declare class PantryService {
         createdAt: Date;
         updatedAt: Date;
         productId: number;
+        location: string | null;
         userId: number;
     }>;
     remove(userId: number, id: number): Promise<{
@@ -54,6 +56,7 @@ export declare class PantryService {
         createdAt: Date;
         updatedAt: Date;
         productId: number;
+        location: string | null;
         userId: number;
     }>;
 }