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/serializers.py

@@ -96,6 +96,7 @@ class TaskSerializer(serializers.ModelSerializer):
             'assigned_to', 'assigned_to_name',
             'vehicle', 'vehicle_registration',
             'work_order', 'work_order_label',
+            'scheduled_date',
             'template_id', 'auto_close_work_order',
             'created_at',
         ]
@@ -106,6 +107,7 @@ class TaskSerializer(serializers.ModelSerializer):
             'status': {'required': False},
             'work_order': {'required': False, 'allow_null': True},
             'vehicle': {'required': False, 'allow_null': True},
+            'scheduled_date': {'required': False, 'allow_null': True},
         }
 
     def validate_title(self, value):