improve checkout loading times

[chrtze]

• create stripe customer whenever a user signs up
  currently, we only create a stripe customer when a user clicks the subscribe button for which we need to wait before showing the checkout. to minimize the loading time of the checkout, we could create the customer already when the user signs up (using hasura events).

• store prices in upstash / redis
  the current version loads the prices each time when the checkout is created. we should cache the prices to make this faster. maybe we can use the stripe webhook to sync the prices with upstash?