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.controller.ts

@@ -1,4 +1,5 @@
 import {
+  Body,
   Controller,
   HttpCode,
   Post,
@@ -13,6 +14,7 @@ import { FileInterceptor } from '@nestjs/platform-express';
 import { memoryStorage } from 'multer';
 import { ReceiptImportService } from './receipt-import.service';
 import { ParsedReceiptItem } from './dto/parsed-receipt-item.dto';
+import { CreateUnitMappingDto } from './dto/create-unit-mapping.dto';
 import { AuthGuard } from '@nestjs/passport';
 
 const ALLOWED_MIMES = [
@@ -61,4 +63,28 @@ export class ReceiptImportController {
     await this.receiptImportService.loadCategories();
     return { message: 'Kategorier har uppdaterats.' };
   }
+
+  @Post('unit-mappings')
+  @UseGuards(AuthGuard('jwt'))
+  async upsertUnitMapping(
+    @Body() dto: CreateUnitMappingDto,
+    @Request() req?: any,
+  ) {
+    const userId =
+      typeof req?.user?.id === 'number'
+        ? req.user.id
+        : typeof req?.user?.userId === 'number'
+          ? req.user.userId
+          : undefined;
+    if (!userId) {
+      throw new BadRequestException('Kunde inte identifiera användaren.');
+    }
+
+    return this.receiptImportService.upsertUnitMapping(
+      userId,
+      dto.productId,
+      dto.originalUnit,
+      dto.preferredUnit,
+    );
+  }
 }