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/core/settings/base.py

@@ -175,6 +175,10 @@ CELERY_BEAT_SCHEDULE = {
         'task': 'modules.fleet.tasks.cleanup_expired_generated_pdfs_task',
         'schedule': crontab(minute=0),
     },
+    'notify-upcoming-tasks-daily': {
+        'task': 'modules.task_management.tasks.notify_upcoming_tasks',
+        'schedule': crontab(hour=8, minute=0),
+    },
 }
 
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