feat: allow application/octet-stream MIME type for PDF uploads in receipt import

Co-authored-by: Copilot <copilot@github.com>

backend/src/receipt-import/receipt-import.controller.ts

@@ -1,5 +1,6 @@
 import {
   Controller,
+  HttpCode,
   Post,
   Request,
   UploadedFile,
@@ -19,12 +20,14 @@ const ALLOWED_MIMES = [
   'image/heic',
   'image/heif',
   'application/pdf',
+  'application/octet-stream', // Flutter Web skickar detta för PDF-filer
 ];
 
 @Controller('receipt-import')
 export class ReceiptImportController {
   constructor(private readonly receiptImportService: ReceiptImportService) {}
 
+  @HttpCode(200)
   @Post()
   @Throttle({ default: { ttl: 60_000, limit: 20 } })
   @UseInterceptors(