Create Portal Session

Open a Stripe Billing Portal session for the logged-in customer.

Used by the account page so paid customers can upgrade / downgrade between Casual / Pro / Business, update card details, and cancel in-app rather than emailing support. The portal's Stripe-side configuration determines exactly which actions are allowed; this endpoint just creates the session URL.

Free users are rejected with 403 — they have no active subscription to manage. The pricing page is the right place to drive an upgrade. The account page surfaces an "Upgrade" CTA instead of a "Manage subscription" button for free users so the 403 path should never normally be hit by the UI.