fix: cabinet purchase fails after panel user deletion — stale UUID

Two bugs caused "RemnaWave UUID не найден" when a user repurchased
after their panel user was deleted (expired user cleanup):

1. Webhook handler only cleared subscription.remnawave_uuid in
   multi-tariff mode. In single-tariff mode the stale UUID remained,
   causing the cabinet to try update_remnawave_user on a deleted
   panel user instead of creating a new one.

2. Cabinet purchase-tariff used subscription.remnawave_uuid for the
   create/update decision. In single-tariff mode this was stale.
   Now mirrors the bot handler logic: checks user.remnawave_uuid
   in single-tariff mode (correctly cleared by webhook).

app/cabinet/routes/subscription_modules/purchase.py

@@ -895,8 +895,14 @@ async def purchase_tariff(
             except Exception as trial_err:
                 logger.warning('Failed to disable trial on RemnaWave', error=trial_err, trial_id=trial_sub.id)
         try:
-            if subscription.remnawave_uuid:
-                # Existing subscription with Remnawave user — update it
+            # Mirror the bot handler logic: in single-tariff mode, check user.remnawave_uuid
+            # (webhook clears it on panel deletion), not subscription.remnawave_uuid
+            if settings.is_multi_tariff_enabled():
+                _should_create = not subscription.remnawave_uuid
+            else:
+                _should_create = not getattr(user, 'remnawave_uuid', None)
+
+            if not _should_create:
                 await service.update_remnawave_user(
                     db,
                     subscription,
@@ -905,7 +911,6 @@ async def purchase_tariff(
                     sync_squads=True,
                 )
             else:
-                # New subscription — create new Remnawave user
                 await service.create_remnawave_user(
                     db,
                     subscription,

app/services/remnawave_webhook_service.py

@@ -1075,8 +1075,8 @@ class RemnaWaveWebhookService:
             subscription.connected_squads = []
             subscription.updated_at = datetime.now(UTC)
 
-            if settings.is_multi_tariff_enabled():
-                subscription.remnawave_uuid = None
+            # Always clear stale UUID — panel user was deleted
+            subscription.remnawave_uuid = None
 
             await db.execute(delete(SubscriptionServer).where(SubscriptionServer.subscription_id == sub_id))