fix(payment): persist amountPaid on payment confirmation

confirmGuestPayment and confirmPayment set the order status to
PROCESSING but never called order.setAmountPaid(...). As a result
amountPaid always read null even after payment, blocking finance
reconciliation against the Swish payout report.

The frontend test mock (GuestPaymentRedirect.spec.ts) expected
amountPaid: 49 but the real backend returned null.

Changes:
  - OrderService: inject app.payment.letter-price (default 49) via
    @Value and set order.setAmountPaid(BigDecimal.valueOf(letterPrice))
    before save in both confirmGuestPayment and confirmPayment
  - OrderServiceTest: assert amountPaid == 49 after both confirm paths

Verified: ./gradlew :backend:test — BUILD SUCCESSFUL

Closes #20

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

@@ -1,6 +1,7 @@
 package se.bilhalsning.service;
 
 import lombok.RequiredArgsConstructor;
+import org.springframework.beans.factory.annotation.Value;
 import org.springframework.stereotype.Service;
 import se.bilhalsning.entity.Order;
 import se.bilhalsning.entity.OrderStatus;
@@ -8,6 +9,7 @@ import se.bilhalsning.exception.InvalidOrderStateException;
 import se.bilhalsning.exception.OrderNotFoundException;
 import se.bilhalsning.repository.OrderRepository;
 
+import java.math.BigDecimal;
 import java.util.List;
 import java.util.UUID;
 
@@ -18,6 +20,9 @@ public class OrderService {
     private final OrderRepository orderRepository;
     private final OrderNotificationService orderNotificationService;
 
+    @Value("${app.payment.letter-price:49}")
+    private int letterPrice = 49;
+
     public Order createOrder(UUID userId, String plate, String letterText) {
         Order order = new Order();
         order.setUserId(userId);
@@ -72,6 +77,7 @@ public class OrderService {
                     "Beställningen kan inte ändras i detta tillstånd");
         }
         order.setStatus(OrderStatus.PROCESSING);
+        order.setAmountPaid(BigDecimal.valueOf(letterPrice));
         Order saved = orderRepository.save(order);
         orderNotificationService.notifyOrderProcessing(saved);
         return saved;
@@ -93,6 +99,7 @@ public class OrderService {
     public Order confirmPayment(UUID orderId, UUID userId) {
         Order order = requirePendingOwnedBy(orderId, userId);
         order.setStatus(OrderStatus.PROCESSING);
+        order.setAmountPaid(BigDecimal.valueOf(letterPrice));
         Order saved = orderRepository.save(order);
         orderNotificationService.notifyOrderProcessing(saved);
         return saved;

backend/src/test/java/se/bilhalsning/service/OrderServiceTest.java

@@ -13,6 +13,7 @@ import se.bilhalsning.exception.InvalidOrderStateException;
 import se.bilhalsning.exception.OrderNotFoundException;
 import se.bilhalsning.repository.OrderRepository;
 
+import java.math.BigDecimal;
 import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
@@ -222,6 +223,7 @@ class OrderServiceTest {
         Order result = orderService.confirmPayment(orderId, userId);
 
         assertEquals(OrderStatus.PROCESSING, result.getStatus());
+        assertEquals(new BigDecimal("49"), result.getAmountPaid());
     }
 
     @Test
@@ -345,6 +347,7 @@ class OrderServiceTest {
         Order result = orderService.confirmGuestPayment(token);
 
         assertEquals(OrderStatus.PROCESSING, result.getStatus());
+        assertEquals(new BigDecimal("49"), result.getAmountPaid());
         verify(orderNotificationService).notifyOrderProcessing(result);
     }