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>

subscription/tests.py

@@ -571,18 +571,16 @@ class YearlyPlanSeedTests(TestCase):
         self.assertEqual((plan.price, plan.ai_daily_quota), (450_000, 9))
         self.assertEqual(Plan.objects.filter(duration_days=30).count(), 1)
 
-    def test_plans_api_reports_each_quota_and_the_free_one(self):
+    def test_only_question_packs_are_on_sale(self):
         from django.core.cache import cache
         cache.clear()
         data = self.client.get('/api/subscription/plans').json()
+        self.assertEqual({p['kind'] for p in data['plans']}, {'pack'})
+        self.assertEqual([p['questionCredits'] for p in data['plans']], [20, 60, 150])
+        self.assertEqual([p['heartsDays'] for p in data['plans']], [7, 30, 60])
         self.assertEqual(data['freeAiDailyQuota'], 3)
-        self.assertEqual({p['aiDailyQuota'] for p in data['plans']}, {10})
-
-    def test_it_is_served_to_the_app(self):
-        from django.core.cache import cache
-        cache.clear()
-        titles = [p['title'] for p in self.client.get('/api/subscription/plans').json()['plans']]
-        self.assertIn('اشتراک یک‌ساله', titles)
+        # پلن‌های اشتراکی پاک نشده‌اند؛ فقط فروخته نمی‌شوند.
+        self.assertFalse(Plan.objects.get(title='اشتراک یک‌ساله').on_sale)
 
     def test_it_never_overwrites_existing_plans(self):
         import importlib