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>

rag/views.py

@@ -152,6 +152,6 @@ class ChunkListView(APIView):
             qs = qs.filter(lesson_id=lesson_id)
         if subject:
             qs = qs.filter(subject=subject)
-        rows = list(qs[:200])
+        rows = list(qs.order_by('-id')[:200])
         data = {'count': len(rows), 'results': ChunkSerializer(rows, many=True).data}
         return Response(ChunkListResponseSerializer(data).data)