fix: use user.user_id instead of user.id in SMTP settings save

CurrentUser object uses user_id attribute, not id. Caused AttributeError
on PUT /api/settings/smtp.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/routers/settings.py

@@ -124,7 +124,7 @@ async def update_smtp_settings(
     if data.smtp_password is not None:
         updates["smtp_password"] = data.smtp_password
 
-    await _set_system_settings(updates, str(user.id))
+    await _set_system_settings(updates, str(user.user_id))
     return {"status": "ok"}