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>

rag/services/answer_cache.py

@@ -35,10 +35,24 @@ def make_key(collection, filter_value, question: str, grade_title: str = '') ->
     یعنی پاسخی که برای دانش‌آموزِ نهم ساخته شده عیناً به دانش‌آموزِ سوم
     برمی‌گشت.
     """
-    raw = f'{collection}|{filter_value}|{(grade_title or "").strip()}|{_norm(question)}'
+    from .generation import PROMPT_VERSION
+
+    # نسخه‌ی پرامپت هم جزءِ کلید است: بعد از عوض شدنِ پرامپت، جوابِ نسخه‌ی
+    # قبلی (مثلاً با LaTeX) نباید تا پایانِ TTL دوباره سرو شود.
+    raw = (f'{PROMPT_VERSION}|{collection}|{filter_value}|'
+           f'{(grade_title or "").strip()}|{_norm(question)}')
     return _PREFIX + hashlib.sha1(raw.encode('utf-8')).hexdigest()
 
 
+def cacheable(history) -> bool:
+    """
+    فقط پیامِ اولِ هر گفتگو کش می‌شود. «نفهمیدم» یا «یه مثال دیگه بزن» معنایش
+    به گفتگوی قبلی بسته است؛ پیش‌تر کلید فقط متنِ سوال بود و هر «نفهمیدم» در
+    همان کتاب، جوابِ کش‌شده‌ی گفتگوی یک نفرِ دیگر را می‌گرفت.
+    """
+    return not history
+
+
 def get(key: str) -> dict | None:
     if not enabled():
         return None