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/tasks.py

@@ -1,10 +1,64 @@
 # backend/modules/task_management/tasks.py
 
+import logging
 from celery import shared_task
 from .models import Task
 from django.contrib.auth import get_user_model
 from modules.fleet.models import Vehicle, WorkOrder
 
+logger = logging.getLogger(__name__)
+
+
+@shared_task
+def notify_upcoming_tasks():
+    """
+    Dnevna Celery periodik task koji šalje Pusher + email notifikacije
+    za radne zadatke čiji je planirani datum izvršenja sutra.
+    """
+    from datetime import date, timedelta
+    from modules.fleet.services import NotificationService
+
+    tomorrow = date.today() + timedelta(days=1)
+    upcoming = Task.objects.filter(
+        is_active=True,
+        scheduled_date=tomorrow,
+        status__in=('aktivan', 'servis'),
+    ).select_related('assigned_to', 'vehicle')
+
+    notified = 0
+    for task in upcoming:
+        recipient = task.assigned_to
+        if not recipient:
+            continue
+        vehicle_label = (
+            task.vehicle.registration_number if task.vehicle else 'nepoznato'
+        )
+        try:
+            NotificationService.create_notification(
+                recipient=recipient,
+                title=f"Sutra planirani zadatak: {task.title}",
+                message=(
+                    f"Radni zadatak '{task.title}' na dizalici {vehicle_label} "
+                    f"je planiran za sutra ({tomorrow.strftime('%d.%m.%Y')})."
+                ),
+                level="info",
+                send_email=True,
+                metadata={
+                    "entity_type": "task",
+                    "task_id": str(task.id),
+                    "vehicle_id": str(task.vehicle_id) if task.vehicle_id else None,
+                    "section": "dashboard",
+                    "scheduled_date": tomorrow.isoformat(),
+                },
+            )
+            notified += 1
+        except Exception:
+            logger.exception("Greška pri slanju notifikacije za task %s", task.id)
+
+    logger.info("notify_upcoming_tasks: obaviješteno %d servisera za datum %s", notified, tomorrow)
+    return {"notified": notified, "date": tomorrow.isoformat()}
+
+
 @shared_task
 def create_task_from_invoice(invoice_data):
     """
@@ -25,7 +79,7 @@ def create_task_from_invoice(invoice_data):
             vehicle = work_order.vehicle
     if vehicle is None and vehicle_id:
         vehicle = Vehicle.objects.filter(pk=vehicle_id).first()
-    
+
     if amount > 1000:
         Task.objects.create(
             title=f"Provjera računa: {invoice_data['invoice_number']}",
@@ -34,4 +88,4 @@ def create_task_from_invoice(invoice_data):
             status='aktivan',
             vehicle=vehicle,
             work_order=work_order,
-        )
+        )