🩹(factory) handle email uniqueness

When generating a batch of users using Faker, there's a possibility of
generating multiple users with the same email address. This breaches
the uniqueness constraint set on the email field, leading to flaky
tests that may fail when random behavior coincides unfavorably.

Implemented a method inspired by Identity's model to ensure unique
email addresses when creating user batches with Faker.
Updated relevant tests for improved stability.

src/backend/core/factories.py

@@ -124,6 +124,7 @@ class UserFactory(factory.django.DjangoModelFactory):
 
     class Meta:
         model = models.User
+        django_get_or_create = ("email",)
 
     email = factory.Faker("email")
     language = factory.fuzzy.FuzzyChoice([lang[0] for lang in settings.LANGUAGES])