🔒️(backend) upgrade Django to 5.2.3 for security compliance

Update Django and related libraries per security auditor recommendations
as current version is aging. Django 5.2.3+ changed email validation per

Remove failing test cases affected by stricter validation.

Refs:
- https://code.djangoproject.com/ticket/36014
- https://github.com/django/django/commit/c068f000

src/backend/core/tests/authentication/test_backends.py

@@ -287,9 +287,7 @@ def test_finds_user_whitespace_email(django_assert_num_queries, settings):
     [
         "john.doe@ｅxample.com",  # Fullwidth character in domain
         "john.doe@еxample.com",  # Cyrillic 'е' in domain
-        "JOHN.DOe@exam𝔭le.com",  # Mixed Gothic '𝔭' in domain
         "john.doe@exаmple.com",  # Cyrillic 'а' (a) in domain
-        "john.doe@ｅ𝓧𝓪𝓶𝓹𝓵𝓮.com",  # Mixed fullwidth and cursive in domain
     ],
 )
 def test_authentication_getter_existing_user_email_tricky(email, monkeypatch, settings):