Include letter text in user-facing order API responses.

Users need the full message on Mina beställningar after creating an
order. The admin API already exposed letterText; user list and payment
confirm endpoints did not.

- Add letterText field to OrderResponse DTO
- Map letterText in OrderController.toResponse and PaymentController.toResponse
- Assert letterText in OrderControllerTest list and create expectations

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

@@ -61,6 +61,7 @@ public class OrderController {
         return new OrderResponse(
                 order.getId(),
                 order.getPlate(),
+                order.getLetterText(),
                 order.getStatus().getValue(),
                 order.getTrackingId(),
                 order.getAmountPaid(),

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

@@ -46,6 +46,7 @@ public class PaymentController {
         return new OrderResponse(
                 order.getId(),
                 order.getPlate(),
+                order.getLetterText(),
                 order.getStatus().getValue(),
                 order.getTrackingId(),
                 order.getAmountPaid(),

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

@@ -7,6 +7,7 @@ import java.util.UUID;
 public record OrderResponse(
         UUID id,
         String plate,
+        String letterText,
         String status,
         String trackingId,
         BigDecimal amountPaid,

backend/src/test/java/se/bilhalsning/controller/OrderControllerTest.java

@@ -83,6 +83,7 @@ class OrderControllerTest {
                 .andExpect(status().isOk())
                 .andExpect(jsonPath("$[0].id").value("c1eebc99-9c0b-4ef8-bb6d-6bb9bd380a11"))
                 .andExpect(jsonPath("$[0].plate").value("ABC123"))
+                .andExpect(jsonPath("$[0].letterText").value("Test letter"))
                 .andExpect(jsonPath("$[0].status").value("sent"))
                 .andExpect(jsonPath("$[0].trackingId").value("PN123456789"));
     }
@@ -130,6 +131,7 @@ class OrderControllerTest {
                 .andExpect(status().isCreated())
                 .andExpect(jsonPath("$.id").value("d1eebc99-9c0b-4ef8-bb6d-6bb9bd380a11"))
                 .andExpect(jsonPath("$.plate").value("ABC123"))
+                .andExpect(jsonPath("$.letterText").value("Hej fin bil!"))
                 .andExpect(jsonPath("$.status").value("pending_payment"));
     }