refactor: add @ManyToOne User relation to Order entity and @EntityGraph query

- Add @ManyToOne(fetch = LAZY) + @JoinColumn(name = "user_id",
  insertable = false, updatable = false) to Order entity so ORM can
  navigate order.getUser().getEmail() for admin responses
- Keep userId as writable UUID field; the relationship is read-only
  to preserve backward compatibility with existing setUserId() calls
- Add getUser() / setUser() accessors
- Replace handwritten @Query JOIN FETCH with Spring Data derived method
  findAllByOrderByCreatedAtDesc() annotated with @EntityGraph(attributePaths
  = {"user"}) — same eager-load behavior, zero custom JPQL
- No database schema change: user_id FK already exists

backend/src/main/java/se/bilhalsning/entity/Order.java

@@ -2,7 +2,10 @@ package se.bilhalsning.entity;
 
 import jakarta.persistence.Column;
 import jakarta.persistence.Entity;
+import jakarta.persistence.FetchType;
 import jakarta.persistence.Id;
+import jakarta.persistence.JoinColumn;
+import jakarta.persistence.ManyToOne;
 import jakarta.persistence.PrePersist;
 import jakarta.persistence.PreUpdate;
 import jakarta.persistence.Table;
@@ -21,6 +24,10 @@ public class Order {
     @Column(name = "user_id", nullable = false, columnDefinition = "uuid")
     private UUID userId;
 
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "user_id", insertable = false, updatable = false)
+    private User user;
+
     @Column(name = "plate", nullable = false, length = 10)
     private String plate;
 
@@ -75,6 +82,14 @@ public class Order {
         this.userId = userId;
     }
 
+    public User getUser() {
+        return user;
+    }
+
+    public void setUser(User user) {
+        this.user = user;
+    }
+
     public String getPlate() {
         return plate;
     }

backend/src/main/java/se/bilhalsning/repository/OrderRepository.java

@@ -1,5 +1,6 @@
 package se.bilhalsning.repository;
 
+import org.springframework.data.jpa.repository.EntityGraph;
 import org.springframework.data.jpa.repository.JpaRepository;
 import org.springframework.stereotype.Repository;
 import se.bilhalsning.entity.Order;
@@ -11,5 +12,9 @@ import java.util.UUID;
 @Repository
 public interface OrderRepository extends JpaRepository<Order, UUID> {
     List<Order> findByUserIdOrderByCreatedAtDesc(UUID userId);
+
     List<Order> findByStatus(OrderStatus status);
+
+    @EntityGraph(attributePaths = {"user"})
+    List<Order> findAllByOrderByCreatedAtDesc();
 }