Manage Billing

Create a Stripe Customer Portal session for plan changes and billing management.

If the request body specifies a target_plan, drop the user directly into the portal's plan-switcher pre-loaded for their active subscription. The in-app Upgrade button uses this so paying customers don't have to navigate the portal homepage to find the plan-change UI. Without target_plan, opens the standard portal homepage (used by Downgrade and the generic billing management link).