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.ts

@@ -162,6 +162,45 @@ export class ReceiptImportService {
     return this.enrichWithAiCategories(matched, userId);
   }
 
+  async upsertUnitMapping(
+    userId: number,
+    productId: number,
+    originalUnit: string,
+    preferredUnit: string,
+  ) {
+    const prismaAny = this.prisma as any;
+    const normalizedOriginalUnit = originalUnit.trim().toLowerCase();
+    const normalizedPreferredUnit = preferredUnit.trim().toLowerCase();
+
+    if (!normalizedOriginalUnit || !normalizedPreferredUnit) {
+      throw new BadRequestException('Enheter måste vara ifyllda.');
+    }
+
+    // Ingen inlärning behövs om enheten redan är samma.
+    if (normalizedOriginalUnit === normalizedPreferredUnit) {
+      return { skipped: true };
+    }
+
+    return prismaAny.unitMapping.upsert({
+      where: {
+        productId_originalUnit_userId: {
+          productId,
+          originalUnit: normalizedOriginalUnit,
+          userId,
+        },
+      },
+      update: {
+        preferredUnit: normalizedPreferredUnit,
+      },
+      create: {
+        productId,
+        userId,
+        originalUnit: normalizedOriginalUnit,
+        preferredUnit: normalizedPreferredUnit,
+      },
+    });
+  }
+
   private async parseReceiptViaImporter(file: Express.Multer.File): Promise<ParsedReceiptItem[]> {
     const form = new FormData();
     form.append(
@@ -205,6 +244,19 @@ export class ReceiptImportService {
     items: ParsedReceiptItem[],
     userId?: number,
   ): Promise<ParsedReceiptItem[]> {
+    type UnitMappingLite = { productId: number; originalUnit: string; preferredUnit: string };
+    type AliasLite = {
+      receiptName: string;
+      product: {
+        id: number;
+        name: string;
+        canonicalName: string | null;
+        categoryRef: { id: number; name: string } | null;
+      };
+    };
+
+    const prismaAny = this.prisma as any;
+
     // Hämta alias och produkter parallellt — filtrera på userId om angivet
     const productFilter = userId ? { isActive: true, ownerId: userId } : { isActive: true };
     const aliasFilter = userId
@@ -215,6 +267,14 @@ export class ReceiptImportService {
           ],
         }
       : { isGlobal: true };
+    const unitMappingsPromise =
+      userId && prismaAny.unitMapping?.findMany
+        ? (prismaAny.unitMapping.findMany({
+            where: { userId },
+            select: { productId: true, originalUnit: true, preferredUnit: true },
+          }) as Promise<UnitMappingLite[]>)
+        : Promise.resolve([] as UnitMappingLite[]);
+
     const [aliases, products, unitMappings] = await Promise.all([
       this.prisma.receiptAlias.findMany({
         where: aliasFilter,
@@ -228,24 +288,27 @@ export class ReceiptImportService {
         where: productFilter,
         select: { id: true, name: true, canonicalName: true, categoryId: true, categoryRef: { select: { id: true, name: true } } },
       }),
-      this.prisma.unitMapping.findMany({
-        where: { userId: userId },
-        select: { productId: true, originalUnit: true, preferredUnit: true },
-      }),
-    ]);
+      unitMappingsPromise,
+    ]) as [AliasLite[], { id: number; name: string; canonicalName: string | null; categoryId: number | null; categoryRef: { id: number; name: string } | null }[], UnitMappingLite[]];
 
     return items.map((item) => {
       const raw = (item.rawName ?? '').toLowerCase().trim();
       if (!raw) return item;
 
       // 1. Alias-match (säker, användaren behöver inte bekräfta)
-      const alias = aliases.find((a) => a.receiptName === raw);
+      const alias = aliases.find((a: AliasLite) => a.receiptName === raw);
       if (alias) {
+        const mappedUnit = unitMappings.find(
+          (um) =>
+            um.productId === alias.product.id &&
+            um.originalUnit === (item.unit ?? '').trim().toLowerCase(),
+        )?.preferredUnit;
         const cat = alias.product.categoryRef;
         return {
           ...item,
           matchedProductId: alias.product.id,
           matchedProductName: alias.product.canonicalName ?? alias.product.name,
+          unit: mappedUnit ?? item.unit,
           ...(cat ? { categorySuggestion: { categoryId: cat.id, categoryName: cat.name, path: cat.name, confidence: 'high' as const, usedFallback: false } } : {}),
         };
       }
@@ -257,7 +320,11 @@ export class ReceiptImportService {
       }
 
       // Kontrollera om det finns en enhetsmappning för produkten och användaren
-      const unitMapping = unitMappings.find((um) => um.productId === suggestion.id && um.originalUnit === item.unit);
+      const unitMapping = unitMappings.find(
+        (um) =>
+          um.productId === suggestion.id &&
+          um.originalUnit === (item.unit ?? '').trim().toLowerCase(),
+      );
       const preferredUnit = unitMapping ? unitMapping.preferredUnit : item.unit;
 
       const cat = suggestion.categoryRef;