cmux-billing

# cmux Billing Read before changing billing, pricing, Stripe, Pro entitlement, checkout, webhook, or subscription code. ## Architecture map - `/api/billing/checkout` creates Stripe Checkout Sessions for Pro when `STRIPE_SECRET_KEY` is set. It sets `client_reference_id` to the Stack user id, auto-creates an anonymous Stack user for signed-out buyers, and falls back to the legacy Stack purchase path when Stripe is unset or `plan=team`. The "already active" short-circuit lives here. - `/api/billing/portal` resolves the current Stack user, looks up their `stripe_customers` row, and creates a Stripe customer portal session returning to `/pricing`. - `/api/billing/subscription` cancels or resumes the active Stripe Pro subscription; `/dashboard/billing` renders localized in-dashboard plan state and self-serve actions. - `web/services/billing/purchase.ts` is the shared idempotent recorder used by `/api/billing/complete` and `/api/stripe/webhook`. It attaches email to the purchaser, records `billing_email_claims` on conflict, and never cross-grants based on an unverified email. - `cmuxPlan` in Stack `clientReadOnlyMetadata` is the only entitlement VM code reads; a `cmuxVmPlan` manual override wins. `resolveProPlanStatus` ORs legacy Stack products with active `stripe_subscriptions` rows. - `/api/stripe/webhook` is signature-verified, insert-first idempotent through `stripe_webhook_events`, safe for foreign events in the shared Stripe account, and gates cmux handling on `metadata.app...