question packs instead of monthly/yearly plans; hearts are real

- packs (20 / 60 / 150 questions for 45,000 / 119,000 / 259,000 toman) never expire and
  give 7 / 30 / 60 days of unlimited hearts; old subscription plans stay for existing
  subscribers but are no longer on sale
- credit ledger (subscription.QuestionCredit): +N per paid pack (once per order), -1 per
  tutor question that reaches the API; the free daily questions are spent first; questions
  stopped by the server filter, cached answers and failed answers cost nothing
- unlimited hearts from packs live in HeartsPass, so buying a pack never rewrites a
  running subscription
- Message.charge records how each question was paid; GET /api/ai/quota and the ask
  responses report free-left-today and credits
- HEARTS_ENFORCED now defaults to on: zero hearts stops answering until refill or a pack

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

ai_tutor/urls.py

@@ -1,9 +1,10 @@
 from django.urls import path
 
-from .views import AskAIView, ConversationMessagesView, MessageFeedbackView
+from .views import AskAIView, ConversationMessagesView, MessageFeedbackView, QuotaView
 
 urlpatterns = [
     path('ai/ask', AskAIView.as_view(), name='ai-ask'),
+    path('ai/quota', QuotaView.as_view(), name='ai-quota'),
     path('ai/conversations/<int:conversation_id>/messages', ConversationMessagesView.as_view(), name='ai-conversation-messages'),
     path('ai/messages/<int:message_id>/feedback', MessageFeedbackView.as_view(), name='ai-message-feedback'),
 ]