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.controller.d.ts

@@ -26,6 +26,7 @@ export declare class PantryController {
         createdAt: Date;
         updatedAt: Date;
         productId: number;
+        location: string | null;
         userId: number;
     })[]>;
     create(user: {
@@ -51,6 +52,7 @@ export declare class PantryController {
         createdAt: Date;
         updatedAt: Date;
         productId: number;
+        location: string | null;
         userId: number;
     }>;
     remove(user: {
@@ -60,6 +62,7 @@ export declare class PantryController {
         createdAt: Date;
         updatedAt: Date;
         productId: number;
+        location: string | null;
         userId: number;
     }>;
 }