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>

content/tests_frontend_contract.py

@@ -67,7 +67,7 @@ class SkillMapTests(TestCase):
         self.assertEqual(rows['ویدیوی توان']['kind'], 'video')
         self.assertEqual(rows['ویدیوی توان']['meta'], '12 دقیقه')
 
-    def test_done_is_false_for_a_guest_and_true_after_completion(self):
+    def test_done_is_false_when_signed_out_and_true_after_completion(self):
         self.assertFalse(any(r['done'] for r in self._lessons()['lessons']))
 
         UserLessonCompletion.objects.create(user_id=self.user.id, lesson=self.quiz,