🐛(backend) fix messaged_at date for import from eml file

Behavior should be identical to other imports (mbox, imap).
We should override created_at of message

src/backend/core/admin.py

@@ -223,7 +223,7 @@ class MessageAdmin(admin.ModelAdmin):
                         # Process EML file synchronously
                         parsed_email = parse_email_message(file_content)
                         if deliver_inbound_message(
-                            str(recipient), parsed_email, file_content
+                            str(recipient), parsed_email, file_content, is_import=True,
                         ):
                             messages.success(
                                 request,

src/backend/core/tests/import/test_import.py

@@ -1,6 +1,8 @@
 """Tests for message import functionality in the admin interface."""
 # pylint: disable=redefined-outer-name, unused-argument
 
+import datetime
+
 from django.core.files.uploadedfile import SimpleUploadedFile
 from django.urls import reverse
 
@@ -101,6 +103,10 @@ def test_import_eml_file(admin_client, eml_file, mailbox):
     assert message.attachments.count() == 1
     assert message.sender.email == "sender@example.com"
     assert message.recipients.get().contact.email == "recipient@example.com"
+    assert message.sent_at == message.thread.messaged_at
+    assert message.sent_at == datetime.datetime(
+        2025, 5, 26, 20, 13, 44, tzinfo=datetime.timezone.utc,
+    )
 
 
 @pytest.mark.django_db
@@ -126,6 +132,9 @@ def test_process_mbox_file_task(mailbox, mbox_file):
     # Check created_at dates match between messages and threads
     assert messages[0].sent_at == messages[0].thread.messaged_at
     assert messages[2].sent_at == messages[1].thread.messaged_at
+    assert messages[2].sent_at == datetime.datetime(
+        2025, 5, 26, 20, 18, 4, tzinfo=datetime.timezone.utc,
+        )
 
     # Check messages
     assert messages[0].subject == "Mon mail avec joli pj"