feat: integritet zatvaranja taska i putnog naloga

Dodaje status 'spreman_za_zavrsetak' i backend provjeru koja blokira zatvaranje taska kada je povezani putni nalog otvoren, uz 409 odgovor i opcionalni auto-close flow.\n\nPri zatvaranju putnog naloga taskovi u statusu 'servis' automatski prelaze u 'spreman_za_zavrsetak'. Frontend modal sada podržava potvrdu i retry s auto_close_work_order=true.\n\nUključeni su i ciljani testovi za novi behavior, te ažuriran prikaz statusa u dashboardu.\n\nCo-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/fleet/services.py

@@ -126,6 +126,7 @@ class VehicleService:
 
     @staticmethod
     def update_work_order(instance: WorkOrder, data: dict) -> WorkOrder:
+        previous_status = instance.status
         for k, v in data.items():
             setattr(instance, k, v)
         try:
@@ -134,8 +135,17 @@ class VehicleService:
             raise ValidationError({"detail": str(e)})
         with transaction.atomic():
             instance.save()
-            # Ako je nalog zatvoren, obavijestimo kreatora/servisera
-            if instance.status == 'closed':
+            transitioned_to_closed = previous_status != 'closed' and instance.status == 'closed'
+            if transitioned_to_closed:
+                from modules.task_management.models import Task
+                Task.objects.filter(
+                    is_active=True,
+                    work_order=instance,
+                    status='servis',
+                ).update(status='spreman_za_zavrsetak')
+
+            # Ako je nalog upravo zatvoren, obavijestimo kreatora/servisera
+            if transitioned_to_closed:
                 try:
                     NotificationService.create_notification(
                         recipient=instance.creator,