feat: add async monthly ZIP generation with notifications

Implement background generation for monthly SN/PN ZIP archives and expose completion through in-app notifications.

- add GeneratedFleetArchive persistence model with expiry metadata
- add archive request/download endpoints and Celery background tasks
- emit notification stages for requested/completed/failed archive jobs
- update calendar bulk download actions to trigger async requests
- add notification modal actions to download generated ZIP files
- extend backend tests for async archive request and download flows

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/modules/fleet/migrations/0035_generatedfleetarchive.py

@@ -0,0 +1,39 @@
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+import modules.fleet.models
+import uuid
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fleet', '0034_monthlyservicerdayentry'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='GeneratedFleetArchive',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, help_text='Unikatni identifikator entiteta (UUID).', primary_key=True, serialize=False)),
+                ('created_at', models.DateTimeField(auto_now_add=True, verbose_name='Vrijeme kreiranja')),
+                ('updated_at', models.DateTimeField(auto_now=True, verbose_name='Vrijeme zadnje izmjene')),
+                ('is_active', models.BooleanField(default=True, verbose_name='Aktivan zapis')),
+                ('archive_type', models.CharField(choices=[('service_tasks', 'Pojedinačni servisni taskovi'), ('work_orders', 'Putni nalozi i računi')], db_index=True, max_length=32, verbose_name='Tip ZIP arhive')),
+                ('year', models.PositiveSmallIntegerField(db_index=True, verbose_name='Godina')),
+                ('month', models.PositiveSmallIntegerField(db_index=True, verbose_name='Mjesec')),
+                ('status', models.CharField(choices=[('pending', 'U obradi'), ('ready', 'Spremno'), ('failed', 'Neuspješno')], db_index=True, default='pending', max_length=16, verbose_name='Status')),
+                ('file', models.FileField(blank=True, null=True, upload_to=modules.fleet.models._generated_fleet_archive_upload_to, verbose_name='ZIP datoteka')),
+                ('filename', models.CharField(blank=True, max_length=255, verbose_name='Naziv datoteke')),
+                ('expires_at', models.DateTimeField(blank=True, db_index=True, null=True, verbose_name='Vrijedi do')),
+                ('error_message', models.TextField(blank=True, verbose_name='Poruka greške')),
+                ('requested_by', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='requested_generated_fleet_archives', to=settings.AUTH_USER_MODEL, verbose_name='Zatražio')),
+            ],
+            options={
+                'verbose_name': 'Generirana ZIP arhiva',
+                'verbose_name_plural': 'Generirane ZIP arhive',
+                'ordering': ['-created_at'],
+            },
+        ),
+    ]