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/task_management/tests/test_views.py

@@ -47,7 +47,12 @@ class TaskViewSetTests(TestCase):
         serializer.instance = task
 
         self.view.perform_update(serializer)
-        mock_update.assert_called_once_with(instance=task, data=serializer.validated_data, user=self.view.request.user)
+        mock_update.assert_called_once_with(
+            instance=task,
+            data=serializer.validated_data,
+            user=self.view.request.user,
+            auto_close_work_order=False,
+        )
         # confirm instance saved with new title
         task.refresh_from_db()
         assert task.title == "Updated"