Billing Checkout Status

The post-return reconcile of a Stripe Checkout Session the browser came back with (?billing=success&session_id=...).

paid is the one field the client dereferences without a null-check: it decides between "Payment successful" and "your payment did not go through". It is True for a COMPLETE Session of the caller whose payment_status is 'paid' or 'no_payment_required' (a $0 first invoice -- credit balance, customer coupon -- is a live subscription too; CHAOS 2026-09-25). A complete Session still 'unpaid' is an async payment PROCESSING: paid is False and status is 'complete', which the client must read as "processing", not "failed". The route never provisions anything -- the webhook owns the tier write -- so a paid answer can precede the tier change by a few seconds.