No LaTeX/markdown anywhere students read (seed, import, DB migration 0021, tutor history, frontend guard); quiz of an island is fixed per user again

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

ai_tutor/tests_plain_history.py

@@ -0,0 +1,25 @@
+from django.core.cache import cache
+from django.test import TestCase
+
+from accounts.authentication import issue_access_token
+from accounts.models import User
+from ai_tutor.models import Conversation, Message
+
+
+class HistoryNeverShowsLatexTests(TestCase):
+    def test_old_latex_message_is_cleaned_on_read(self):
+        cache.clear()
+        user = User.objects.create(phone='09120000042')
+        conv = Conversation.objects.create(user_id=user.id)
+        Message.objects.create(conversation=conv, role='user', text='حل کن $x^{2}=4$')
+        Message.objects.create(conversation=conv, role='assistant', text=r'جواب: \( \frac{1}{2} \vec{a} \) و **مهم** x^2')
+        auth = {'HTTP_AUTHORIZATION': f'Bearer {issue_access_token(user.id)}'}
+        r = self.client.get(f'/api/ai/conversations/{conv.id}/messages', **auth)
+        self.assertEqual(r.status_code, 200, r.content)
+        texts = [m['text'] for m in r.json()['messages']]
+        for t in texts:
+            self.assertNotIn('\\', t)
+            self.assertNotIn('**', t)
+            self.assertNotIn('^', t)
+            self.assertNotIn('$', t)
+        self.assertIn('x²', texts[0])