Hi,
The Joomla administration is secured ; that is why Nicolas gave you a tip in order to be automatically login.
Otherwise, any call in the backend will be refused (because it need a login), like you have with your 403 answer.
The other solution would be to create a cron plugin to perform the category rebuild.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.