harden OTP for real SMS and close the second audit round

Kavenegar is being switched on, so every OTP request now costs credit:
- phone numbers are normalised to 09XXXXXXXXX everywhere (login, OTP,
  family invite, TEST_LOGIN_PHONES keys) and existing rows are migrated;
  non-Iranian and landline numbers are rejected before any SMS
- cooldown and a daily per-phone cap are counted in the database, so the
  three gunicorn workers no longer triple them
- a global daily OTP SMS cap (default 300) stops SMS pumping
- per-IP rates widened for CGNAT / school Wi-Fi (env-overridable)
- provider message id is logged; SMS text warns not to share the code

Second audit round: an offer no longer shortens a longer paid plan,
Zarinpal error bodies no longer 500, island XP reaches the parent report,
a non-numeric questionId no longer 500s after the answer is generated.

596 tests green.

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

subscription/services.py

@@ -65,16 +65,29 @@ def grant_free_subscription(user_id: int, *, duration_days: int | None = None, e
 
     مستقل از caller اتمیک است. ردیف Subscription قبل از خواندن/تغییر با
     select_for_update قفل می‌شود تا فراخوانی‌های هم‌زمان lost update نسازند.
+
+    اشتراکِ فعالی که دیرتر تمام می‌شود (یا نامحدود است) کوتاه نمی‌شود؛ آفر فقط
+    وقتی اثر دارد که بیشتر از چیزی که کاربر دارد بدهد. پیش از ۲۰۲۶-۰۹-۲۵ کاربری
+    که سالانه خریده بود و بعد کدِ آفرِ ۷ روزه می‌زد (یا مهمانی که خریده بود و بعد
+    ثبت‌نام می‌کرد و آفرِ خودکارِ ثبت‌نام می‌گرفت) با ۷ روز می‌ماند.
     """
     with transaction.atomic():
         Subscription.objects.get_or_create(user_id=user_id)
         subscription = Subscription.objects.select_for_update().get(user_id=user_id)
         old_status = subscription.status
 
-        subscription.status = 'active'
-        subscription.expires_at = (
+        offered_until = (
             timezone.now() + timezone.timedelta(days=duration_days) if duration_days else None
         )
+        still_active = old_status == 'active' and (
+            subscription.expires_at is None or subscription.expires_at > timezone.now()
+        )
+        keeps_longer = still_active and offered_until is not None and (
+            subscription.expires_at is None or subscription.expires_at >= offered_until
+        )
+        subscription.status = 'active'
+        if not keeps_longer:
+            subscription.expires_at = offered_until
         subscription.save(update_fields=['status', 'expires_at'])
 
         status_changed.send(