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>

content/serializers.py

@@ -51,6 +51,7 @@ class UnitWithNodesSerializer(serializers.Serializer):
     id = serializers.CharField()
     title = serializers.CharField()
     blurb = serializers.CharField(allow_blank=True)
+    requiresSubscription = serializers.BooleanField(help_text='این کاربر برای بازیِ این فصل اشتراک لازم دارد (فقط فصلِ اولِ هر پایه رایگان است).')
     nodes = NodeStatusSerializer(many=True)