improve one-time payment webhook logic

[vincanger]

Description

Fixes #371

Adds a new webhook event handler for payment_intent.succeeded. One-time payment products (i.e. "credits"), characterized by Stripe as having the payment mode (not the subscription mode), do not send an invoice.paid event (although subscriptions do), so we have to confirm payment and handle it better than just using the checkout.session.completed event, which can be sent even in the case that a payment fails.

Contributor Checklist

Make sure to do the following steps if they are applicable to your PR:

• Update e2e tests: If you changed the /template/app, then make sure to do any neccessary updates to /template/e2e-tests also.
• Update demo app: If you changed the /template/app, then make sure to do any neccessary updates to /opensaas-sh/app_diff also. Check /opensaas-sh/README.md for details.
• Update docs: If needed, update the /opensaas-sh/blog/src/content/docs.

[infomiho]

If I understood correctly, we want to give users access when we are sure that payment succeeded. That's why we introduced a new hook payment_intent.succeeded.

I've left some comments.

[infomiho]

Suggested change

	const metadata = returnMetadataByMode({ mode, priceId });
	const paymentIntentData = getPaymentIntentData({ mode, priceId });

Related to the suggested fn rename below.

[infomiho]

Here it feels like we are implicitly saying: we only want to update the subscriptionPlan value for subscription plans by not using the numOfCreditsPurchased value here. Can we be explicit about it? Something like:

if (plan.effect.kind === 'credits') {
  return;
}

[infomiho]

Good job 👍