fix: recover stale pending monthly ZIP requests

Mark stale pending fleet archives as failed after a timeout window and create a fresh request so users receive completion notifications and can download ZIP files again.

Add regression coverage for replacing stale pending service-task archive requests.

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

backend/modules/fleet/tests/test_work_order_images_endpoint.py

@@ -561,6 +561,35 @@ class WorkOrderImagesEndpointTests(TestCase):
         self.assertIsNotNone(generated)
         self.assertEqual(generated.status, 'pending')
 
+
+    def test_monthly_service_tasks_archive_request_replaces_stale_pending_archive(self):
+        stale_pending = GeneratedFleetArchive.objects.create(
+            requested_by=self.user,
+            archive_type='service_tasks',
+            year=2033,
+            month=12,
+            status='pending',
+            filename='pending-archive.zip',
+            expires_at=timezone.now() + timedelta(days=2),
+        )
+        stale_created_at = timezone.now() - timedelta(minutes=10)
+        GeneratedFleetArchive.objects.filter(pk=stale_pending.pk).update(created_at=stale_created_at)
+
+        response = self.client.post(
+            '/api/fleet/reports/monthly-service-tasks-archive-request/',
+            data={'year': 2033, 'month': 12},
+            format='json',
+        )
+        self.assertIn(response.status_code, [200, 202], response.content)
+        payload = response.json()
+        self.assertIn('generated_archive_id', payload)
+        self.assertNotEqual(str(stale_pending.pk), str(payload['generated_archive_id']))
+
+        stale_pending.refresh_from_db()
+        self.assertFalse(stale_pending.is_active)
+        self.assertEqual(stale_pending.status, 'failed')
+        self.assertIn('pending statusu', stale_pending.error_message)
+
     def test_monthly_work_orders_archive_request_creates_ready_download_with_notification(self):
         WorkOrderInvoice.objects.create(
             work_order=self.work_order,