feat: dodaj scheduled_date na Task i kalendar widget

- Novo polje Task.scheduled_date (DateField, nullable, db_index) za planiranje
- Migracija 0009_task_scheduled_date
- Celery task notify_upcoming_tasks: dnevno u 08:00 šalje Pusher + email za sutrašnje zadatke
- TaskCalendarWidget: react-spring slide panel s lijeve strane, grid prikaz mjeseca, dot indikatori, klik otvara TaskServiceRecordsModal
- FleetDashboardShell: import + render kalendara, Planirano stupac u tablici zadataka
- TaskCreateModal + TaskServiceRecordsModal: scheduled_date input i prikaz
- Admin + serializer + views: scheduled_date filter i prikaz
- 47 backend testova prolaze (uključuje 8 novih za notify_upcoming_tasks)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/task_management/tests/test_serializers.py

@@ -43,4 +43,41 @@ class TaskSerializerTests(TestCase):
         task = Task.objects.create(title="Task 4", description="", status="servis", assigned_to=self.user, vehicle=self.vehicle)
 
         serializer = TaskSerializer(instance=task, data={"status": "zavrsen", "work_order": str(work_order.id)}, partial=True)
-        assert serializer.is_valid(), serializer.errors
+        assert serializer.is_valid(), serializer.errors
+
+    def test_scheduled_date_is_optional_on_create(self):
+        serializer = TaskSerializer(data={"title": "Task planned", "status": "aktivan", "vehicle": str(self.vehicle.id)})
+        assert serializer.is_valid(), serializer.errors
+        assert 'scheduled_date' not in serializer.errors
+
+    def test_scheduled_date_is_included_in_serialized_output(self):
+        import datetime
+        task = Task.objects.create(
+            title="Task with date",
+            assigned_to=self.user,
+            vehicle=self.vehicle,
+            scheduled_date=datetime.date(2026, 9, 1),
+        )
+        serializer = TaskSerializer(instance=task)
+        assert 'scheduled_date' in serializer.data
+        assert str(serializer.data['scheduled_date']) == '2026-09-01'
+
+    def test_scheduled_date_can_be_null_in_serialized_output(self):
+        task = Task.objects.create(title="Task no date", assigned_to=self.user, vehicle=self.vehicle)
+        serializer = TaskSerializer(instance=task)
+        assert serializer.data['scheduled_date'] is None
+
+    def test_scheduled_date_accepts_valid_date(self):
+        serializer = TaskSerializer(
+            data={"title": "Task mit datum", "status": "aktivan",
+                  "vehicle": str(self.vehicle.id), "scheduled_date": "2026-12-25"}
+        )
+        assert serializer.is_valid(), serializer.errors
+
+    def test_scheduled_date_rejects_invalid_date_string(self):
+        serializer = TaskSerializer(
+            data={"title": "Task bad date", "status": "aktivan",
+                  "vehicle": str(self.vehicle.id), "scheduled_date": "not-a-date"}
+        )
+        assert not serializer.is_valid()
+        assert 'scheduled_date' in serializer.errors