chore: expose prod frontend on host port 3001

Add ports mapping 3001:80 to the prod frontend service so the application
is accessible from the server at http://srvr.nu:3001 for testing before
DNS is pointed to bilhej.se. Backend remains internal-only (no host port).

docker-compose.prod.yml

@@ -39,6 +39,8 @@ services:
       dockerfile: docker/frontend.prod.Dockerfile
       context: .
     container_name: bilhej-frontend-prod
+    ports:
+      - "3001:80"
     depends_on:
       - backend
     networks: