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/tests_quota.py

@@ -30,7 +30,7 @@ class DailyQuotaTests(TestCase):
         self.assertEqual([self.ask().status_code for _ in range(2)], [200, 200])
         blocked = self.ask()
         self.assertEqual(blocked.status_code, 429)
-        self.assertIn('اشتراک', blocked.json()['detail'])
+        self.assertIn('بسته', blocked.json()['detail'])
 
     def test_remaining_quota_follows_the_users_own_limit(self):
         self.assertEqual(self.ask().json()['remainingQuota'], 1)