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/serializers.py

@@ -12,6 +12,9 @@ class PlanSerializer(serializers.Serializer):
     features = serializers.ListField(child=serializers.CharField())
     badge = serializers.CharField(allow_blank=True)
     aiDailyQuota = serializers.IntegerField(help_text='سوال در روز از دستیار برای مشترکِ این پلن.')
+    kind = serializers.ChoiceField(choices=['pack', 'subscription'])
+    questionCredits = serializers.IntegerField(help_text='بسته: سوال‌هایی که به اعتبار اضافه می‌شود (منقضی نمی‌شود).')
+    heartsDays = serializers.IntegerField(help_text='بسته: روزهای قلبِ نامحدودِ هدیه.')
 
 
 class PlansListResponseSerializer(serializers.Serializer):
@@ -33,6 +36,8 @@ class PurchaseResponseSerializer(serializers.Serializer):
 
 
 class SubscriptionStatusResponseSerializer(serializers.Serializer):
-    active = serializers.BooleanField()
+    active = serializers.BooleanField(help_text='اشتراکِ زمانی (ماهانه/سالانه‌ی قدیمی)؛ نه بسته.')
     planId = serializers.CharField(allow_null=True)
     expiresAt = serializers.DateTimeField(allow_null=True)
+    questionCredits = serializers.IntegerField(help_text='سوال‌های خریده‌شده‌ی باقی‌مانده (منقضی نمی‌شوند).')
+    unlimitedHeartsUntil = serializers.DateTimeField(allow_null=True, help_text='پایانِ قلبِ نامحدودِ هدیه‌ی بسته.')