feat: dodaj display_code polje za putne naloge (MT070626 format)

- Dodano polje 'display_code' na WorkOrder model (format: 2 inicijala + ddmmyy)
- Migracije za kreiranje polja i backfill postojećih naloga
- Ažuriran WorkOrderSerializer za validaciju i serijalizaciju display_code
- Backend tasks za auto-generiranje display_code pri kreiranju
- Frontend komponente ažurirane za prikaz display_code umjesto UUID:
  * FleetDashboardShell tablica putnih naloga
  * WorkOrderDetailModal
  * TaskCreateModal
  * Sve PDF stranice
- displayIds.js utility za formatiranjeDisplay ID-eva
- Admin interface prikazuje display_code

backend/modules/fleet/tests/test_services.py

@@ -3,7 +3,9 @@ from django.test import TestCase
 from django.contrib.auth import get_user_model
 from modules.fleet.services import VehicleService
 from modules.fleet.models import Vehicle, WorkOrder, VehicleServiceRecord
+from modules.fleet.serializers import WorkOrderSerializer
 from modules.task_management.models import Task
+from rest_framework.test import APIRequestFactory
 
 
 class FleetServiceTests(TestCase):
@@ -72,4 +74,58 @@ class FleetServiceTests(TestCase):
 
         VehicleService.update_work_order(work_order, {"status": "closed"})
         task.refresh_from_db()
-        self.assertEqual(task.status, "spreman_za_zavrsetak")
+        self.assertEqual(task.status, "spreman_za_zavrsetak")
+
+    def test_closing_work_order_moves_active_task_to_ready_for_completion(self):
+        work_order = WorkOrder.objects.create(
+            vehicle=self.vehicle,
+            creator=self.user,
+            purpose="kontrola",
+            status="open",
+            start_mileage=5000,
+            end_mileage=5200,
+        )
+        task = Task.objects.create(
+            title="Preventiva",
+            description="",
+            status="aktivan",
+            assigned_to=self.user,
+            vehicle=self.vehicle,
+            work_order=work_order,
+        )
+
+        VehicleService.update_work_order(work_order, {"status": "closed"})
+        task.refresh_from_db()
+        self.assertEqual(task.status, "spreman_za_zavrsetak")
+
+    def test_work_order_serializer_autogenerates_display_code(self):
+        self.user.first_name = "Mario"
+        self.user.last_name = "Tkalac"
+        self.user.save(update_fields=["first_name", "last_name"])
+
+        request = APIRequestFactory().post('/api/fleet/work-orders/', {})
+        request.user = self.user
+        serializer = WorkOrderSerializer(
+            data={
+                "vehicle": str(self.vehicle.id),
+                "purpose": "kontrola",
+            },
+            context={"request": request},
+        )
+        self.assertTrue(serializer.is_valid(), serializer.errors)
+        work_order = serializer.save()
+        self.assertRegex(work_order.display_code, r'^MT[0-9]{6}$')
+
+    def test_work_order_serializer_rejects_invalid_display_code(self):
+        request = APIRequestFactory().post('/api/fleet/work-orders/', {})
+        request.user = self.user
+        serializer = WorkOrderSerializer(
+            data={
+                "vehicle": str(self.vehicle.id),
+                "purpose": "kontrola",
+                "display_code": "BAD-CODE",
+            },
+            context={"request": request},
+        )
+        self.assertFalse(serializer.is_valid())
+        self.assertIn("display_code", serializer.errors)