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/admin.py

@@ -26,8 +26,8 @@ class TaskTemplateEntryInline(admin.TabularInline):
 
 @admin.register(Task)
 class TaskAdmin(admin.ModelAdmin):
-    list_display = ('title', 'status', 'assigned_to', 'vehicle', 'work_order', 'created_at')
-    list_filter = ('status', 'created_at', 'work_order')
+    list_display = ('title', 'status', 'scheduled_date', 'assigned_to', 'vehicle', 'work_order', 'created_at')
+    list_filter = ('status', 'scheduled_date', 'created_at', 'work_order')
     search_fields = ('title', 'description', 'work_order__id', 'work_order__vehicle__registration_number')
     readonly_fields = ('id', 'created_at', 'updated_at')
     inlines = (TaskWorkHoursTableInline, TaskWorkEntryInline)