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>

content/receivers.py

@@ -1,7 +1,7 @@
 from django.db.models.signals import post_delete, post_save
 from django.dispatch import receiver
 
-from .models import Grade, Lesson, Node, Unit, UserAnswer, UserLessonCompletion
+from .models import Grade, Lesson, Node, Question, Unit, UserAnswer, UserLessonCompletion
 from .services import touch_node_progress_on_answer, update_node_progress_on_lesson_completion
 
 
@@ -29,6 +29,10 @@ def _on_lesson_completion_created(sender, instance, created, **kwargs):
 @receiver(post_delete, sender=Node, dispatch_uid='content_bump_curriculum_node_delete')
 @receiver(post_save, sender=Lesson, dispatch_uid='content_bump_curriculum_lesson_save')
 @receiver(post_delete, sender=Lesson, dispatch_uid='content_bump_curriculum_lesson_delete')
+# سوال هم: نقشه فقط درس‌های سوال‌دار را برای تمام شدنِ جزیره می‌شمارد، پس
+# اولین سوالِ یک درس (از ادمین یا import) باید نقشه‌ی کش‌شده را باطل کند.
+@receiver(post_save, sender=Question, dispatch_uid='content_bump_curriculum_question_save')
+@receiver(post_delete, sender=Question, dispatch_uid='content_bump_curriculum_question_delete')
 def _bump_curriculum_cache_version(sender, **kwargs):
     from .views import bump_curriculum_version
     bump_curriculum_version()