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/tasks.py

@@ -27,7 +27,7 @@ from pytesseract import TesseractNotFoundError
 from reportlab.lib.pagesizes import A4
 from reportlab.lib.utils import ImageReader
 from reportlab.pdfgen import canvas
-from .models import VehicleNotification, GeneratedWorkOrderPdf
+from .models import VehicleNotification, GeneratedWorkOrderPdf, GeneratedFleetArchive
 from .pdf_layout import register_unicode_fonts, draw_standard_header_footer
 from .email_utils import append_user_signature
 
@@ -855,6 +855,26 @@ def cleanup_expired_generated_pdfs_task():
     return {"deleted": deleted}
 
 
+@shared_task
+def cleanup_expired_generated_archives_task():
+    now = timezone.now()
+    expired = GeneratedFleetArchive.objects.filter(
+        is_active=True,
+        expires_at__isnull=False,
+        expires_at__lte=now,
+    )
+    deleted = 0
+    for item in expired:
+        if item.file:
+            item.file.delete(save=False)
+        item.is_active = False
+        item.status = 'failed'
+        item.error_message = 'ZIP arhiva je istekla.'
+        item.save(update_fields=['is_active', 'status', 'error_message', 'updated_at'])
+        deleted += 1
+    return {"deleted": deleted}
+
+
 @shared_task
 def build_work_order_pdf_cached_task(generated_pdf_id):
     from .views import _build_work_order_pdf, _build_work_order_service_records_pdf
@@ -934,4 +954,98 @@ def build_work_order_pdf_cached_task(generated_pdf_id):
                 },
             )
         logger.exception("Greška kod build_work_order_pdf_cached_task: %s", exc)
-        return {"status": "failed", "error": str(exc)}
+        return {"status": "failed", "error": str(exc)}
+
+
+@shared_task
+def build_monthly_archive_cached_task(generated_archive_id):
+    from .services import NotificationService
+    from .views import (
+        _build_monthly_service_tasks_archive_content,
+        _build_monthly_work_orders_archive_content,
+        _generated_archive_filename_for_user,
+    )
+
+    generated = (
+        GeneratedFleetArchive.objects
+        .select_related('requested_by')
+        .filter(pk=generated_archive_id, is_active=True)
+        .first()
+    )
+    if generated is None:
+        return {'error': 'Generated ZIP zapis nije pronađen.'}
+
+    requested_by = generated.requested_by
+    if requested_by is None:
+        generated.status = 'failed'
+        generated.error_message = 'Korisnik koji je zatražio ZIP arhivu nije dostupan.'
+        generated.save(update_fields=['status', 'error_message', 'updated_at'])
+        return {'status': 'failed', 'error': generated.error_message}
+
+    try:
+        if generated.archive_type == 'work_orders':
+            archive_bytes = _build_monthly_work_orders_archive_content(
+                user=requested_by,
+                year=generated.year,
+                month=generated.month,
+            )
+        else:
+            archive_bytes = _build_monthly_service_tasks_archive_content(
+                user=requested_by,
+                year=generated.year,
+                month=generated.month,
+            )
+
+        filename = generated.filename or _generated_archive_filename_for_user(
+            requested_by,
+            year=generated.year,
+            month=generated.month,
+            archive_type=generated.archive_type,
+        )
+        generated.file.save(filename, ContentFile(archive_bytes), save=False)
+        generated.status = 'ready'
+        generated.error_message = ''
+        generated.save(update_fields=['file', 'status', 'error_message', 'updated_at'])
+
+        NotificationService.create_notification(
+            recipient=requested_by,
+            title='ZIP arhiva spremna',
+            message=f"ZIP arhiva je spremna za preuzimanje ({generated.month:02d}.{generated.year}.).",
+            level='success',
+            send_email=False,
+            metadata={
+                'entity_type': 'fleet_archive',
+                'archive_type': generated.archive_type,
+                'stage': 'completed',
+                'year': generated.year,
+                'month': generated.month,
+                'generated_archive_id': str(generated.pk),
+                'download_url': f"fleet/reports/generated-archives/{generated.pk}/download/",
+                'filename': generated.filename or filename,
+                'expires_at': generated.expires_at.isoformat() if generated.expires_at else None,
+                'section': 'service-records',
+            },
+        )
+        return {'status': 'ready', 'generated_archive_id': str(generated.pk)}
+    except Exception as exc:
+        generated.status = 'failed'
+        generated.error_message = str(exc)
+        generated.save(update_fields=['status', 'error_message', 'updated_at'])
+        NotificationService.create_notification(
+            recipient=requested_by,
+            title='Greška kod ZIP arhive',
+            message=f"Generiranje ZIP arhive nije uspjelo ({generated.month:02d}.{generated.year}.).",
+            level='warning',
+            send_email=False,
+            metadata={
+                'entity_type': 'fleet_archive',
+                'archive_type': generated.archive_type,
+                'stage': 'failed',
+                'year': generated.year,
+                'month': generated.month,
+                'generated_archive_id': str(generated.pk),
+                'section': 'service-records',
+            },
+        )
+        logger.exception("Greška kod build_monthly_archive_cached_task: %s", exc)
+        return {'status': 'failed', 'error': str(exc)}