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/models.py

@@ -47,6 +47,13 @@ class Task(BaseModel):
         related_name='work_order_tasks',
         verbose_name=_("Putni nalog"),
     )
+    scheduled_date = models.DateField(
+        null=True,
+        blank=True,
+        db_index=True,
+        verbose_name=_("Planirani datum izvršenja"),
+        help_text=_("Datum kada je planirano izvršenje radnog zadatka."),
+    )
 
     class Meta:
         ordering = ['-created_at']