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/models.py

@@ -25,6 +25,11 @@ class Message(models.Model):
     role = models.CharField(max_length=10, choices=ROLE_CHOICES)
     text = models.TextField()
     question_id = models.IntegerField(null=True, blank=True)  # ارجاع سبک به content.Question
+    # فقط برای پیامِ کاربر: این سوال از کجا پرداخت شد. «free» از سهمیه‌ی روزانه، «credit» از
+    # اعتبارِ بسته (subscription.QuestionCredit)، «none» هیچ (رد در صافیِ سرور یا جوابِ کش‌شده که
+    # به API نرسید). سهمیه‌ی رایگانِ امروز = تعدادِ «free»های امروز.
+    CHARGE_CHOICES = [('free', 'رایگانِ روزانه'), ('credit', 'اعتبارِ بسته'), ('none', 'بی‌هزینه')]
+    charge = models.CharField(max_length=6, choices=CHARGE_CHOICES, blank=True, default='')
     created_at = models.DateTimeField(auto_now_add=True)
 
     # --- حاشیه‌نویسیِ تولید (فقط برای پیام‌های role='assistant') --------------