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/views.py

@@ -17,6 +17,8 @@ from rest_framework.views import APIView
 
 from accounts.models import User as AccountUser
 from config.db import release_db_connection
+from content.plain_math import convert as plain_convert
+from rag.services.mathtext import to_plain
 from . import concurrency, provider_status
 from .models import AskIdempotency, Conversation, Message, MessageFeedback
 from .serializers import (
@@ -823,7 +825,7 @@ class AskAIView(APIView):
         data = {
             'conversationId': str(conversation.id),
             'messageId': str(existing.assistant_message.id),
-            'answer': existing.assistant_message.text,
+            'answer': to_plain(existing.assistant_message.text),
             'suggestedFollowUps': existing.suggested_follow_ups,
             'remainingQuota': state['remainingQuota'],
             'credits': state['credits'],
@@ -852,7 +854,9 @@ class ConversationMessagesView(APIView):
             'conversationId': str(conversation.id),
             'lessonId': str(conversation.lesson_id) if conversation.lesson_id else None,
             'messages': [
-                {'messageId': str(m.id), 'role': m.role, 'text': m.text, 'diagram': m.diagram}
+                {'messageId': str(m.id), 'role': m.role,
+                 # تورِ ایمنی: پیامِ قدیمی با LaTeX/markdown هم هرگز خام نمایش داده نمی‌شود.
+                 'text': to_plain(m.text) if m.role == 'assistant' else plain_convert(m.text), 'diagram': m.diagram}
                 for m in messages
             ],
         }