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>

accounts/authentication.py

@@ -71,6 +71,14 @@ def decode_access_token(token: str) -> int:
     return int(value)
 
 
+def _issued_at(token: str) -> int:
+    """
+    ثانیه‌ی صدورِ توکن. قالبِ TimestampSigner «مقدار:زمان:امضا» است و زمان
+    base62؛ فقط بعد از unsignِ موفق صدا زده می‌شود، پس ساختار معتبر است.
+    """
+    return signing.b62_decode(token.rsplit(':', 2)[-2])
+
+
 class TadrisinoTokenAuthentication(BaseAuthentication):
     """
     هدر مورد انتظار: Authorization: Bearer <access_token>
@@ -91,7 +99,10 @@ class TadrisinoTokenAuthentication(BaseAuthentication):
 
         token = parts[1]
         user_id = decode_access_token(token)
-        return (_get_request_user(user_id), token)
+        user = _get_request_user(user_id)
+        if user.tokens_valid_after is not None and _issued_at(token) < int(user.tokens_valid_after.timestamp()):
+            raise AuthenticationFailed('از حساب خارج شده‌اید؛ دوباره وارد شوید.')
+        return (user, token)
 
     def authenticate_header(self, request):
         return self.keyword