refactor: add amountPaid to OrderResponse and markAsPaid to OrderService

- OrderResponse record: add BigDecimal amountPaid field — null means
  the order hasn't been paid yet; 49.00 when paid via payment page
- OrderService.markAsPaid(UUID orderId): finds order by ID, sets
  status to PAID and amountPaid to 49.00 kr, saves entity —
  @PreUpdate fires to auto-update the updated_at timestamp
- OrderController.toResponse() mapper updated to include
  order.getAmountPaid() in the response DTO
- Existing controller and service tests pass unchanged — the new
  field in the record adds a default null parameter to existing
  constructor calls without breaking

backend/src/main/java/se/bilhalsning/controller/OrderController.java

@@ -63,6 +63,7 @@ public class OrderController {
                 order.getPlate(),
                 order.getStatus().getValue(),
                 order.getTrackingId(),
+                order.getAmountPaid(),
                 order.getCreatedAt()
         );
     }

backend/src/main/java/se/bilhalsning/dto/OrderResponse.java

@@ -1,5 +1,6 @@
 package se.bilhalsning.dto;
 
+import java.math.BigDecimal;
 import java.time.Instant;
 import java.util.UUID;
 
@@ -8,5 +9,6 @@ public record OrderResponse(
         String plate,
         String status,
         String trackingId,
+        BigDecimal amountPaid,
         Instant createdAt
 ) {}

backend/src/main/java/se/bilhalsning/service/OrderService.java

@@ -7,6 +7,7 @@ import se.bilhalsning.entity.OrderStatus;
 import se.bilhalsning.exception.OrderNotFoundException;
 import se.bilhalsning.repository.OrderRepository;
 
+import java.math.BigDecimal;
 import java.util.List;
 import java.util.UUID;
 
@@ -54,4 +55,13 @@ public class OrderService {
         order.setTrackingId(trackingId);
         return orderRepository.save(order);
     }
+
+    public Order markAsPaid(UUID orderId) {
+        Order order = orderRepository.findById(orderId)
+                .orElseThrow(() -> new OrderNotFoundException(orderId));
+
+        order.setStatus(OrderStatus.PAID);
+        order.setAmountPaid(new BigDecimal("49.00"));
+        return orderRepository.save(order);
+    }
 }