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>

accounts/tests_otp_limits.py

@@ -36,7 +36,7 @@ class PhoneNormalizationTests(TestCase):
         self.assertEqual(set(OTPCode.objects.values_list('phone', flat=True)), {'09121112233'})
 
     def test_login_with_another_spelling_reaches_the_same_account(self):
-        User.objects.create(phone='09121112244', is_guest=False)
+        User.objects.create(phone='09121112244')
         with patch('accounts.views.secrets.randbelow', return_value=246810), \
              patch('accounts.views.send_otp_sms'):
             _request('+989121112244')