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>

payment/tests_security_review.py

@@ -30,7 +30,7 @@ def client_for(user):
 class _Base(TestCase):
     def setUp(self):
         cache.clear()
-        self.user = User.objects.create(phone='09120009901', is_guest=False)
+        self.user = User.objects.create(phone='09120009901')
         self.app = client_for(self.user)
         self.pack = Plan.objects.get(kind='pack', question_credits=20)