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/fleet/models.py

@@ -250,6 +250,7 @@ class WorkOrder(BaseModel):
         ('defektaza', _("Defektaža")),
         ('kontrola', _("Kontrola")),
         ('redovni_pregled', _("Redovni pregled")),
+        ('popravak', _("Popravak")),
     ]
 
     vehicle = models.ForeignKey(Vehicle, on_delete=models.PROTECT, related_name='work_orders', verbose_name=_("Vozilo"))