fix race condition in stripe webhook for newer API users

[vincanger]

userStripeId is undefined on the invoice.paid event in newer API versions due to a likely race condition (i.e. checkout.session.completed not being called first)

the fix would be to pull the users from the stripe event on each event trigger https://github.com/wasp-lang/open-saas/blob/main/app/src/server/webhooks/stripe.ts#L84

    } else if (event.type === 'invoice.paid') {

      userStripeId = session.customer as string;

      const invoice = event.data.object as Stripe.Invoice;
      const periodStart = new Date(invoice.period_start * 1000);
      await context.entities.User.updateMany({
        where: {
          stripeId: userStripeId,
        },
        data: {
          hasPaid: true,
          datePaid: periodStart,
        },
      });