Security review fixes: admin permissions, guest sessions, payment edge cases, assistant leaks

- Admin: refund/cancel/mark-resolved/Bazaar-link actions need change permission; question and
  curriculum import need add/delete; content browser needs view_question; guest device ids are
  shown to superusers only.
- Accounts: registering a guest's phone revokes the guest's earlier sessions; self-deletion keeps
  Redemption, UserOffer and the phone's OTP rows; the public demo guest cannot be deleted
  (DEMO_GUEST_DEVICE_IDS); display-name contact filter covers any separator and social handles.
- Payment: callback without Authority is 400; a second Bazaar purchase on a paid order is 409;
  orders snapshot credits/hearts/days at creation (payment.0011).
- Assistant: a broken diagram no longer makes a streamed answer free; a late off-topic marker
  refuses the answer (not stored, not cached); a disconnect saves the partial answer; only
  KNOWN_COLLECTIONS are searched.
- Infra: secret-key file is created atomically (workers no longer split keys); no default
  INTERNAL_SERVICE_KEY; Kavenegar key and Bazaar token kept out of error logs; Swagger/ReDoc
  pinned; schema throttled; docker compose bound to 127.0.0.1; non-ASCII digit ids are 400;
  ticket text capped at 5000.
- docs/security-before-production.md: remaining open items 24-31 and the guard tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -100,8 +100,10 @@ BAZAAR_REFRESH_TOKEN=
 # ═══════════════════════════════════════════════════════════════════════
 # ۶) اختیاری — پیش‌فرض‌ها معمولاً کافی‌اند
 # ═══════════════════════════════════════════════════════════════════════
-# کلیدِ مسیرهای داخلیِ سرور-به-سرور؛ خالی روی سرور = آن مسیرها بسته‌اند.
+# کلیدِ مسیرهای داخلیِ سرور-به-سرور؛ خالی = آن مسیرها بسته‌اند (در DEBUG هم پیش‌فرضی ندارد).
 # INTERNAL_SERVICE_KEY=
+# مهمان‌های نمایشی که با «حذفِ حساب» پاک نمی‌شوند (با کاما). پیش‌فرض: frontend-test-device.
+# DEMO_GUEST_DEVICE_IDS=
 # با صفر قلب جلوی جواب دادن گرفته شود؟ پیش‌فرض روشن (از ۲۰۲۶-۱۰-۰۲)؛ ۰ خاموشش می‌کند.
 # HEARTS_ENFORCED=1
 # سقف‌های کدِ ورود: هر شماره در روز، و کلِ سامانه در روز (ضدِ SMS pumping).