When generating a GPT response, the demo AI app checks whether a user is subscribed with a flat-rate subscription or has remaining credits.

If the user is eligible, it subtracts their credits in both cases. It should instead only spend credits if there isn't an active flat-rate subscription.

The should probably check for credits only if PaymentPlan is 10credits.

Context: https://github.com/wasp-lang/open-saas/pull/391/files#r1979841748