Confirm subscription upgrade with Stripe

Confirms the subscription upgrade process for the organization's Stripe-based subscription.

Called by the frontend after the Stripe SetupIntent process is completed, as notification to the backend that the pending StripeSubscription can now be considered the active subscription for the organization.

This is a UX optimization so that the user will see the pending state immediately after completing the SetupIntent, instead of having to wait for the Stripe webhook to be processed. The upgrade will still be finalized properly by the webhook handler even if the call to this endpoint fails or is never made. This endpoint is idempotent and returns 204 without changes when the webhook has already activated and linked this subscription.