throttle hot read endpoints and cap unbounded list responses

progress/views.py
  - GuestAwareReadThrottle (same guest-by-IP pattern as the other apps);
    throttle_scope='read' on UserSummaryView, StreakCalendarView,
    LeaderboardView.
notifications/views.py
  - GuestAwareReadThrottle + throttle_scope='read' on NotificationListView.
  - list capped at NOTIFICATIONS_MAX=100 newest (Meta.ordering already
    newest-first); unreadCount still counted over the full set.
notifications/tests.py
  - 150 rows -> 100 returned newest-first, unreadCount = 75.
rag/views.py
  - ChunkListView: explicit order_by('-id') before the existing [:200] cap.
ai_tutor/views.py
  - ConversationMessagesView: return the newest 200 messages in
    chronological order instead of the whole conversation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ai_tutor/views.py

@@ -511,7 +511,9 @@ class ConversationMessagesView(APIView):
     @extend_schema(responses=ConversationMessagesResponseSerializer, tags=['AI Tutor (ai_tutor)'])
     def get(self, request, conversation_id):
         conversation = get_object_or_404(Conversation, pk=conversation_id, user_id=request.user.id)
-        messages = conversation.messages.all()
+        # آخرین ۲۰۰ پیام، به ترتیبِ زمانی (قدیمی→جدید). جلوی رشدِ نامحدودِ پاسخ
+        # برای یک مکالمه‌ی خیلی طولانی رو می‌گیره.
+        messages = reversed(list(conversation.messages.order_by('-id')[:200]))
 
         data = {
             'conversationId': str(conversation.id),