✅(backend) fix randomly failing test on user search

The user account created to query the API had a random email
that could randomly interfere with our search results.

src/backend/core/tests/test_api_users.py

@@ -186,7 +186,7 @@ def test_api_users_list_query_short_queries():
     """
     Queries shorter than 5 characters should return an empty result set.
     """
-    user = factories.UserFactory()
+    user = factories.UserFactory(email="paul@example.com")
     client = APIClient()
     client.force_login(user)