but looks like you will have to keep all your customers' wallets on the server, because bitcoin core does not support watch only wallets. Kinda strange, because if you have all blockchain why not to allow to peek at a wallet balance like block explorer or blockchain.info does? I want to be able to notify a user immediately of his payment, I don't care if he double-spend, I can deal with it later

I was thinking to have user's wallets as cold and just watch them, but now I am rethinking...