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/serializers.py

@@ -34,7 +34,8 @@ class AskResponseSerializer(serializers.Serializer):
     messageId = serializers.CharField()
     answer = serializers.CharField()
     suggestedFollowUps = serializers.ListField(child=serializers.CharField())
-    remainingQuota = serializers.IntegerField()
+    remainingQuota = serializers.IntegerField(help_text='سوال‌های رایگانِ باقیِ امروز.')
+    credits = serializers.IntegerField(required=False, help_text='اعتبارِ بسته‌های سوال (منقضی نمی‌شود).')
     diagram = serializers.DictField(required=False, allow_null=True)
     # تکه‌های درسیِ بازیابی‌شده از Qdrant که پاسخ بر پایه‌شون ساخته شده؛
     # خالی یعنی retrieval انجام نشده (نبودِ subject/lessonNumber یا خاموش‌بودنِ RAG).
@@ -66,3 +67,9 @@ class FeedbackRequestSerializer(serializers.Serializer):
 
 class FeedbackResponseSerializer(serializers.Serializer):
     success = serializers.BooleanField()
+
+
+class QuotaResponseSerializer(serializers.Serializer):
+    freeDaily = serializers.IntegerField(help_text='سوال‌های رایگانِ هر روز برای این کاربر.')
+    remainingQuota = serializers.IntegerField(help_text='سوال‌های رایگانِ باقیِ امروز.')
+    credits = serializers.IntegerField(help_text='اعتبارِ بسته‌های سوال؛ منقضی نمی‌شود.')