ASAT Expiration Time Extended from 48h to 1 week

[mlajkim]

Background

Last time we have implemented automatic renewal of access token when end user (or FE) calls /auth/prep API (#53)
As I have multiple devices and use them to review word cards of the Wordnote, it would be cool to have a longer valid token so that I do not have to sign in every time I use.

TODOs

• Decide the expiring time (probably 1 week?)
• Change the title of the issue according to the decision of the new expiring time.
• Implement
• Release