fix: invalidate stale service records PDF cache on service record changes

Ensure admin/API deletes and updates of service records, photos, and
attachments invalidate the cached service-records PDF. Also harden cached
PDF lookup/download paths so a missing file is treated as stale cache
instead of a 500.

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

backend/modules/fleet/tests/test_work_order_images_endpoint.py

@@ -298,6 +298,52 @@ class WorkOrderImagesEndpointTests(TestCase):
         self.assertFalse(cached.is_active)
         self.assertEqual(cached.status, 'failed')
 
+    def test_service_record_delete_invalidates_service_records_pdf_cache(self):
+        cached = GeneratedWorkOrderPdf.objects.create(
+            work_order=self.work_order,
+            requested_by=self.user,
+            pdf_type='service_records',
+            status='ready',
+            filename='MT150726.work-order-service-records.pdf',
+            expires_at=timezone.now() + timedelta(hours=1),
+        )
+        cached.file.save(
+            'MT150726.work-order-service-records.pdf',
+            ContentFile(b'%PDF-1.4 cached service records'),
+            save=True,
+        )
+
+        VehicleServiceRecord.objects.filter(pk=self.service_record.pk).delete()
+
+        cached.refresh_from_db()
+        self.assertFalse(cached.is_active)
+        self.assertEqual(cached.status, 'failed')
+        self.assertFalse(cached.file.name)
+
+    def test_service_records_pdf_rebuilds_when_cached_file_is_missing(self):
+        cached = GeneratedWorkOrderPdf.objects.create(
+            work_order=self.work_order,
+            requested_by=self.user,
+            pdf_type='service_records',
+            status='ready',
+            filename='MT150726.work-order-service-records.pdf',
+            expires_at=timezone.now() + timedelta(hours=1),
+        )
+        cached.file.save(
+            'MT150726.work-order-service-records.pdf',
+            ContentFile(b'%PDF-1.4 cached service records'),
+            save=True,
+        )
+        cached.file.storage.delete(cached.file.name)
+
+        response = self.client.get(f"/api/fleet/work-orders/{self.work_order.pk}/service-records-pdf/")
+        self.assertEqual(response.status_code, 200, response.content)
+        self.assertEqual(response['Content-Type'], 'application/pdf')
+
+        cached.refresh_from_db()
+        self.assertFalse(cached.is_active)
+        self.assertEqual(cached.status, 'failed')
+
     def test_service_records_docx_contains_embedded_service_photos(self):
         VehicleServicePhoto.objects.create(
             service_record=self.service_record,