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/models.py

@@ -21,6 +21,10 @@ class User(models.Model):
     # مهمانِ واقعی). فقط برای هدف‌گیریِ آینده‌ی کمپین‌های پیامی نگه‌داری
     # می‌شه؛ در حال حاضر هیچ منطقِ دیگه‌ای به این فیلد وابسته نیست.
     last_activity_at = models.DateTimeField(null=True, blank=True)
+    # access tokenهای صادرشده پیش از این لحظه رد می‌شوند (خروج از حساب). توکنِ
+    # access فقط user_id امضاشده است و به Session وصل نیست؛ بدونِ این، خروج
+    # فقط refresh را باطل می‌کرد و توکنِ فعلی تا یک ساعت کار می‌کرد.
+    tokens_valid_after = models.DateTimeField(null=True, blank=True)
 
     class Meta:
         ordering = ['-created_at']
@@ -74,6 +78,9 @@ class OTPCode(models.Model):
     code = models.CharField(max_length=6)
     expires_at = models.DateTimeField()
     is_used = models.BooleanField(default=False)
+    # تلاش‌های غلط روی همین کد. در دیتابیس شمرده می‌شود، نه در کشِ LocMem که
+    # برای هر worker جداست؛ بعد از OTP_MAX_FAILED_ATTEMPTS کد باطل می‌شود.
+    failed_attempts = models.PositiveSmallIntegerField(default=0)
 
     def __str__(self):
         return f"otp-{self.phone}"