Billing reliability: credit purchases fulfill exactly once
- Credit pack purchases now credit exactly once, even when Stripe redelivers the same webhook
- The billing page verifies every purchase after checkout, so customers get their credits even if a webhook is lost
- AI credit spending is atomic under concurrent calls, and the usage card reports a truthful state when no plan is assigned
- New regression tests pin the critical auth, tenancy, and billing behaviors, and CI now fails if the generated API client drifts from the backend
