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/src/receipt-import/receipt-import.service.spec.ts

@@ -40,6 +40,7 @@ describe('ReceiptImportService test matrix', () => {
     category: { findMany: jest.fn().mockResolvedValue([]) },
     receiptAlias: { findMany: jest.fn().mockResolvedValue([]) },
     product: { findMany: jest.fn().mockResolvedValue([]) },
+    unitMapping: { findMany: jest.fn().mockResolvedValue([]) },
   };
 
   const aiServiceMock = {
@@ -60,6 +61,7 @@ describe('ReceiptImportService test matrix', () => {
     jest.clearAllMocks();
     prismaMock.receiptAlias.findMany.mockResolvedValue([]);
     prismaMock.product.findMany.mockResolvedValue([]);
+    prismaMock.unitMapping.findMany.mockResolvedValue([]);
   });
 
   describe('ignore patterns', () => {
@@ -222,5 +224,37 @@ describe('ReceiptImportService test matrix', () => {
       expect(second[0].matchedProductName).toBe('Mjolk');
       expect(second[0].suggestedProductId).toBeUndefined();
     });
+
+    it('använder inlärd enhetsmappning vid aliasträff', async () => {
+      prismaMock.receiptAlias.findMany.mockResolvedValue([
+        {
+          receiptName: 'mjolk 1l',
+          productId: 501,
+          product: {
+            id: 501,
+            name: 'Mjolk user',
+            canonicalName: 'Mjolk user',
+            categoryId: 30,
+            categoryRef: { id: 30, name: 'Mejeri' },
+          },
+        },
+      ]);
+
+      prismaMock.unitMapping.findMany.mockResolvedValue([
+        {
+          productId: 501,
+          originalUnit: 'l',
+          preferredUnit: 'st',
+        },
+      ]);
+
+      const result = await (service as any).matchProducts(
+        [{ rawName: 'MJOLK 1L', unit: 'L' }],
+        77,
+      );
+
+      expect(result[0].matchedProductId).toBe(501);
+      expect(result[0].unit).toBe('st');
+    });
   });
 });