Add user_login field and migrate existing credentials.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/apps/accounts/tests_user_login.py

@@ -0,0 +1,23 @@
+from django.contrib.auth import authenticate
+from django.test import TestCase
+
+from apps.accounts.models import User
+
+
+class UserLoginFieldTests(TestCase):
+    def test_authenticate_uses_user_login_not_user_name(self):
+        User.objects.create_user(
+            user_login="gm.login",
+            password="gm123",
+            user_name="gm",
+            status=User.STATUS_GM,
+        )
+        self.assertIsNotNone(
+            authenticate(username="gm.login", password="gm123")
+        )
+        self.assertIsNone(authenticate(username="gm", password="gm123"))
+
+    def test_create_user_defaults_user_name_to_login(self):
+        user = User.objects.create_user(user_login="solo", password="x")
+        self.assertEqual(user.user_login, "solo")
+        self.assertEqual(user.user_name, "solo")