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/serializers.py

@@ -37,22 +37,6 @@ class CreateOrderResponseSerializer(serializers.Serializer):
     provider = serializers.CharField()
 
 
-class WebhookRequestSerializer(serializers.Serializer):
-    # کال‌بک از سمت درگاه پرداخت میاد، نه از اپ
-    orderId = serializers.CharField()
-    gateway = serializers.CharField(required=False, default='zarinpal')
-    gatewayRefId = serializers.CharField(required=False, allow_blank=True)
-    # ⚠️ اصلاح امنیتی: قبلاً status یک CharField آزاد بود و هر مقداری
-    # (حتی مقادیر پشتیبانی‌نشده/اشتباه) رو مستقیم روی Order ذخیره می‌کرد.
-    # حالا فقط دو وضعیت نهاییِ معتبرِ کال‌بک درگاه پذیرفته می‌شه؛ 'pending'
-    # وضعیت اولیه‌ی داخلیه و نباید از بیرون توسط webhook قابل‌تنظیم باشه.
-    status = serializers.ChoiceField(choices=['paid', 'failed'])
-
-
-class WebhookResponseSerializer(serializers.Serializer):
-    received = serializers.BooleanField()
-
-
 class ZarinpalCallbackResponseSerializer(serializers.Serializer):
     """فقط وقتی settings.ZARINPAL_REDIRECT_URL خالی باشه برگردونده می‌شه؛
     وگرنه کاربر به همون URL ریدایرکت می‌شه، نه به این JSON."""