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/migrations/0009_task_scheduled_date.py

@@ -0,0 +1,22 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('task_management', '0008_task_add_ready_for_completion_status'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='task',
+            name='scheduled_date',
+            field=models.DateField(
+                null=True,
+                blank=True,
+                db_index=True,
+                verbose_name='Planirani datum izvršenja',
+                help_text='Datum kada je planirano izvršenje radnog zadatka.',
+            ),
+        ),
+    ]