tutor answers step by step in plain text, islands finish, and the audit's money and login holes are closed

Tutor (prompt tutor-v3-stepwise-plain): plain text instead of LaTeX/markdown,
with rag/services/mathtext.py converting whatever still slips through; full
step-by-step solutions with a check before the final answer; three tones by
grade; the last 6 messages of the conversation go to the model and follow-ups
are no longer served from another user's cache; book solutions the checker
disputes are kept out of the context; a questionId gives the tutor the
practice question and the student's wrong answer. /api/health reports when
the AI credit runs out. rag_model_benchmark compares chat models on verified
book problems (run on the server; credit ran out mid-run).

Map: only lessons that have questions count toward finishing an island,
lessonIds[0] / nextLessonId is always the next lesson to play, and a finished
island stays finished. The Seyar characters (sigma/delta/pi) are seeded.

Audit fixes, each reproduced before fixing:
- the plan is stored on the order, so paying a monthly order after starting
  a yearly checkout no longer grants a year
- opening checkout no longer demotes an active subscription; renewals extend
  from the current expiry; expired subscriptions are not reported active
- one discount code per order and only before the gateway; a 100% code
  completes the purchase; lost Zarinpal confirmations are re-verified
- the unused /api/payment/webhook "mark paid" endpoint is removed
- XP counts only answers recorded on the server
- 6-digit OTP, each code dies after 5 wrong guesses; POST /api/auth/logout
- a guest who signs up keeps their account and progress (test phones excluded)
- affiliate commission on the first real payment; guests excluded
- parents with several children no longer get a 500
- SQLite runs transactions IMMEDIATE, since it ignores select_for_update
- admin login is rate-limited; display names drop phone numbers and links;
  userQuestion is capped at 2000 characters
- server-side heart enforcement exists behind HEARTS_ENFORCED (off until the
  frontend handles 403 no_hearts)

Not done on purpose: rotating the server's secret key and moving to Postgres.

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

payment/permissions.py

@@ -1,3 +1,5 @@
+import secrets
+
 from django.conf import settings
 from rest_framework.permissions import BasePermission
 
@@ -14,18 +16,7 @@ class HasInternalServiceKey(BasePermission):
 
     def has_permission(self, request, view):
         provided = request.headers.get('X-Internal-Key', '')
-        return bool(provided) and provided == settings.INTERNAL_SERVICE_KEY
-
-
-class HasWebhookSecret(BasePermission):
-    """
-    برای POST /api/payment/webhook: باید از سمت درگاه پرداخت بیاد، نه یک
-    کلاینت دلخواه. تا زمان اتصال درگاه واقعی (که خودش امضای HMAC داره)،
-    یک shared-secret ساده از طریق هدر X-Webhook-Secret بررسی می‌شه.
-    """
-
-    message = 'امضای وب‌هوک نامعتبر است.'
-
-    def has_permission(self, request, view):
-        provided = request.headers.get('X-Webhook-Secret', '')
-        return bool(provided) and provided == settings.PAYMENT_WEBHOOK_SECRET
+        expected = settings.INTERNAL_SERVICE_KEY
+        # مقایسه‌ی زمان‌ثابت: == در اولین نویسه‌ی متفاوت برمی‌گردد و از زمانِ پاسخ
+        # می‌شود کلید را نویسه‌به‌نویسه حدس زد.
+        return bool(provided) and bool(expected) and secrets.compare_digest(provided, expected)