create book transfers and ACH credits via Column's bank API

Steps

1. Authenticate requests with your Column API key using HTTP Basic auth (api_key as the username, empty password), scoped to sandbox or production based on the subdomain.
2. Create bank accounts for your customers by POSTing to /bank-accounts with the required entity (person or business) relationship and account type.
3. For internal book transfers (between two Column accounts at the same bank), POST to /transfers/book with amount (in cents), currency, source bank account ID, destination bank account ID, and a description.
4. For outbound ACH credits, POST to /transfers/ach with the source bank_account_id, amount, counterparty details (routing number, account number, account type), direction (credit), and an optional same_day boolean.
5. Retrieve transfer status from GET /transfers/ach/{id} or receive updates via webhooks configured in the Column dashboard.
6. Test returns and other failure scenarios in the sandbox using Column's simulation endpoints before moving to production.