- Add upsert_billing_customer / get_billing_customer (billing_customers table) - Add record_transaction (idempotent on provider_transaction_id) - Fix upsert_subscription: remove provider_customer_id param, key by provider_subscription_id instead of user_id (allows multi-sub) - Update webhook handler to call upsert_billing_customer separately Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>