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/models.py

@@ -65,6 +65,16 @@ def _generated_work_order_pdf_upload_to(instance, filename):
     return f"fleet/{vehicle_reg}/generated_pdfs/{unique_name}"
 
 
+def _generated_fleet_archive_upload_to(instance, filename):
+    requester = 'unknown'
+    try:
+        requester = str(instance.requested_by_id or 'unknown')
+    except Exception:
+        requester = 'unknown'
+    unique_name = f"{uuid.uuid4().hex}_{filename}"
+    return f"fleet/generated_archives/{requester}/{unique_name}"
+
+
 class VehicleServicePhoto(BaseModel):
     """
     Fotografije kvarova / radova povezane s VehicleServiceRecord.
@@ -480,6 +490,59 @@ class GeneratedWorkOrderPdf(BaseModel):
         return f"{self.pdf_type}:{self.work_order_id}:{self.status}"
 
 
+class GeneratedFleetArchive(BaseModel):
+    ARCHIVE_TYPE_CHOICES = [
+        ('service_tasks', _("Pojedinačni servisni taskovi")),
+        ('work_orders', _("Putni nalozi i računi")),
+    ]
+    STATUS_CHOICES = [
+        ('pending', _("U obradi")),
+        ('ready', _("Spremno")),
+        ('failed', _("Neuspješno")),
+    ]
+
+    requested_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        null=True,
+        blank=True,
+        on_delete=models.SET_NULL,
+        related_name='requested_generated_fleet_archives',
+        verbose_name=_("Zatražio"),
+    )
+    archive_type = models.CharField(
+        max_length=32,
+        choices=ARCHIVE_TYPE_CHOICES,
+        db_index=True,
+        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(
+        max_length=16,
+        choices=STATUS_CHOICES,
+        default='pending',
+        db_index=True,
+        verbose_name=_("Status"),
+    )
+    file = models.FileField(
+        upload_to=_generated_fleet_archive_upload_to,
+        null=True,
+        blank=True,
+        verbose_name=_("ZIP datoteka"),
+    )
+    filename = models.CharField(max_length=255, blank=True, verbose_name=_("Naziv datoteke"))
+    expires_at = models.DateTimeField(null=True, blank=True, db_index=True, verbose_name=_("Vrijedi do"))
+    error_message = models.TextField(blank=True, verbose_name=_("Poruka greške"))
+
+    class Meta:
+        verbose_name = _("Generirana ZIP arhiva")
+        verbose_name_plural = _("Generirane ZIP arhive")
+        ordering = ['-created_at']
+
+    def __str__(self):
+        return f"{self.archive_type}:{self.year}-{self.month:02d}:{self.status}"
+
+
 class VehicleServiceRecord(BaseModel):
     """
     Zapisi o servisima/popravcima na vozilu.