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/serializers.py

@@ -32,7 +32,20 @@ class NodeStatusSerializer(serializers.Serializer):
     status = serializers.ChoiceField(choices=['locked', 'unlocked', 'completed'])
     # شناسه‌ی درس‌های همین گره تا کلاینت بدونه با کلیک روی گره کدوم درس رو
     # باز کنه (endpointهای lessons/{id}/... مستقیم با همین id کار می‌کنن).
-    lessonIds = serializers.ListField(child=serializers.IntegerField())
+    # ⚠️ عضوِ اول همیشه nextLessonId است؛ بقیه به ترتیبِ کتاب. فرانتِ فعلی با
+    # کلیک روی جزیره lessonIds[0] را باز می‌کند، پس با همین ترتیب هر بار درسِ
+    # بعدی باز می‌شود — پیش‌تر همیشه درسِ اول باز می‌شد و جزیره هرگز تمام نمی‌شد.
+    lessonIds = serializers.ListField(
+        child=serializers.IntegerField(),
+        help_text='عضوِ اول = nextLessonId؛ بقیه به ترتیبِ کتاب.',
+    )
+    nextLessonId = serializers.IntegerField(
+        allow_null=True,
+        help_text=(
+            'درسی که با کلیک روی این جزیره باید باز شود: اولین درسِ تمام‌نشده‌ای '
+            'که سوال دارد؛ اگر همه تمام شده‌اند، اولین درس (برای مرور).'
+        ),
+    )
 
 
 class UnitWithNodesSerializer(serializers.Serializer):