✅(backend) fix randomly failing test due to delay before check

The signature is issued when the request is made but we only check the
timestamp after a few other lines of code/checks. In some cases,
this was causing a 1s difference in the timestamp (because the request
was made at the just a fraction of a second before the next instruction...)
and the test was failing:

> assert response["X-Amz-Date"] == timezone.now().strftime("%Y%m%dT%H%M%SZ")
AssertionError: assert equals failed '20250504T175307Z'  '20250504T175308Z'

Author: sampaccoud

src/backend/core/tests/documents/test_api_documents_duplicate.py

@@ -5,6 +5,7 @@
 import base64
 import uuid
 from io import BytesIO
+from unittest import mock
 from urllib.parse import urlparse
 
 from django.conf import settings
@@ -133,19 +134,21 @@ def test_api_documents_duplicate_success(index):
 
     # Ensure access persists after the owner loses access to the original document
     models.DocumentAccess.objects.filter(document=document).delete()
-    response = client.get(
-        "/api/v1.0/documents/media-auth/", HTTP_X_ORIGINAL_URL=image_refs[0][1]
-    )
 
-    assert response.status_code == 200
+    now = timezone.now()
+    with mock.patch("django.utils.timezone.now", return_value=now):
+        response = client.get(
+            "/api/v1.0/documents/media-auth/", HTTP_X_ORIGINAL_URL=image_refs[0][1]
+        )
 
+    assert response.status_code == 200
+    assert response["X-Amz-Date"] == now.strftime("%Y%m%dT%H%M%SZ")
     authorization = response["Authorization"]
     assert "AWS4-HMAC-SHA256 Credential=" in authorization
     assert (
         "SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature="
         in authorization
     )
-    assert response["X-Amz-Date"] == timezone.now().strftime("%Y%m%dT%H%M%SZ")
 
     s3_url = urlparse(settings.AWS_S3_ENDPOINT_URL)
     response = requests.get(