Remove the guest system entirely: no is_guest/device_id, no guest throttles, phone login is the only way in

accounts.0008 drops the columns and refuses to run while guest rows exist.
Docs describe the current state; tests that only proved guest behaviour are gone.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

config/tests.py

@@ -171,7 +171,7 @@ class APIRequestLoggingMiddlewareTests(TestCase):
         self.client = APIClient()
 
     def test_authenticated_api_request_is_logged_with_user_id_and_no_secrets(self):
-        user = User.objects.create(device_id='device-mw-log-1', is_guest=True, name='')
+        user = User.objects.create(phone='09000000005', name='')
         token = issue_access_token(user.id)
 
         from content.models import Grade