🚨(backend) fix linter warnings

Recent updates of dev/ruff and dev/pylint dependencies led
to new linting warnings.

Pylint 3.2.0 introduced a new check `possibly-used-before-assignment`,
which ensures variables are defined regardless of conditional statements.

Some if/else branches were missing defaults. These have been fixed.

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

@@ -92,9 +92,12 @@ def test_models_oidc_user_getter_invalid_token(django_assert_num_queries, monkey
 
     monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
 
-    with django_assert_num_queries(0), pytest.raises(
-        SuspiciousOperation,
-        match="User info contained no recognizable user identification",
+    with (
+        django_assert_num_queries(0),
+        pytest.raises(
+            SuspiciousOperation,
+            match="User info contained no recognizable user identification",
+        ),
     ):
         klass.get_or_create_user(access_token="test-token", id_token=None, payload=None)