Paywall: first unit of each grade free; 1/3/6-month subscriptions; AI sold separately

- content: unit_is_free/has_unit_access; questions, node quiz, answer and lesson/node
  complete return 402 subscription_required outside the first unit without an active
  subscription. /api/home units carry requiresSubscription.
- subscription.0013: monthly 498k, quarterly 798k, six-month 1,298k toman, unlimited
  hearts, 3 AI questions/day; packs stay on sale for more questions.
- AI_DAILY_QUOTA_FREE default 3 -> 2.
- /api/subscription/status adds planTitle, planDurationDays, aiDailyQuota.
- tests: content/tests_paywall.py; existing unlock-order tests get a subscription.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

subscription/serializers.py

@@ -43,7 +43,10 @@ class PurchaseResponseSerializer(serializers.Serializer):
 
 
 class SubscriptionStatusResponseSerializer(serializers.Serializer):
-    active = serializers.BooleanField(help_text='اشتراکِ زمانی (ماهانه/سالانه‌ی قدیمی)؛ نه بسته.')
+    active = serializers.BooleanField(help_text='اشتراکِ زمانی فعال (یک/سه/شش‌ماهه)؛ نه بسته.')
+    planTitle = serializers.CharField(allow_null=True, help_text='نامِ پلنِ اشتراکِ فعال؛ null بی‌اشتراک.')
+    planDurationDays = serializers.IntegerField(allow_null=True, help_text='طولِ دوره‌ی پلنِ فعال به روز.')
+    aiDailyQuota = serializers.IntegerField(help_text='سقفِ سوالِ روزانه‌ی همین کاربر از دستیار.')
     planId = serializers.CharField(allow_null=True)
     expiresAt = serializers.DateTimeField(allow_null=True)
     questionCredits = serializers.IntegerField(help_text='سوال‌های خریده‌شده‌ی باقی‌مانده (منقضی نمی‌شوند).')