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>

.env.example

@@ -85,7 +85,7 @@ QDRANT_API_KEY=
 # OPENAI_EMBEDDING_MODEL=text-embedding-3-small
 # سقفِ سوالِ روزانه از دستیار برای کاربرِ بدونِ اشتراک (پیش‌فرض ۳). سقفِ مشترکان روی
 # هر پلن در ادمین است (ai_daily_quota). حسابِ هزینه: docs/ai-cost-and-guardrails.md
-# AI_DAILY_QUOTA_FREE=3
+# AI_DAILY_QUOTA_FREE=2
 
 # ═══════════════════════════════════════════════════════════════════════
 # ۵) پرداخت