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>

accounts/tests_audit_fixes.py

@@ -131,7 +131,9 @@ class HeartsTests(TestCase):
         return _client(self.user).post(f'/api/questions/{self.question.id}/answer',
                                        {'selectedOptionId': self.right.id}, format='json')
 
-    def test_off_by_default_so_the_current_app_keeps_working(self):
+    # از ۲۰۲۶-۱۰-۰۲ پیش‌فرض روشن است (قلبِ هدیه‌ی بسته‌ها بدونِ آن اثری نداشت)؛ با .env خاموش می‌شود.
+    @override_settings(HEARTS_ENFORCED=False)
+    def test_can_be_switched_off(self):
         self.assertEqual(self._answer().status_code, 200)
 
     @override_settings(HEARTS_ENFORCED=True)