BillingPortalController

Subscriber-facing REST endpoints for the native membership billing portal. Ships in feature/built-in-membership-portal-emails. Uses cookie authentication — no admin nonce required. Exposes endpoints for reading subscription and invoice data (GET /account/billing-portal), creating SetupIntents for payment method updates, updating payment methods, cancelling subscriptions at period end, and resuming pending cancellations. All write endpoints reflect changes on the local subscription record immediately without waiting for a webhook round-trip. Delegates all Stripe API calls to StripeSubscriptionManager.