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>

ai_tutor/tests.py

@@ -444,6 +444,8 @@ class AskAIViewThrottleTests(TestCase):
             HTTP_IDEMPOTENCY_KEY=idempotency_key,
         )
 
+    # سقفِ روزانه (پیش‌فرض ۲) نباید پیش از throttle برسد؛ این تست فقط throttle را می‌سنجد.
+    @override_settings(AI_DAILY_QUOTA_FREE=10)
     def test_requests_within_rate_succeed_and_excess_is_throttled(self):
         # هر درخواست idempotency-key متفاوت دارد تا هر بار واقعاً از کوتای
         # روزانه مصرف کند (نه یک replay ساده)، و صرفاً throttle سنجیده شود.