Creates an empty bank payroll in draft state. The request body takes a
name and the sender_id (UUID of the source bank_account), plus the
optional online flag (true — the default — for same-day payrolls,
false for deferred), payment_date (required when online is false)
and category.

Line items are not part of this request — attach them by creating each
transaction via POST /v1/banking/bank_transactions with the new payroll's
id passed as bank_payroll_id.

Once the line items are in place, walk the lifecycle:

1. POST /v1/banking/bank_payrolls/{id}/enroll — mandatory first step;
   requires status: draft, no transition in flight, and at least one
   attached transaction. The call answers 200 while the payroll is still
   transition: enroll; the bank settles it asynchronously into either
   enrolled or pending_validation (the branch step 2 covers), or
   enroll_failed. Poll the show endpoint for the landing state.
2. POST /v1/banking/bank_payrolls/{id}/validate — conditional branch,
   only when enrollment left the payroll in pending_validation (recipients
   the bank still needs validated). Requires token and
   verification_code.
3. POST /v1/banking/bank_payrolls/{id}/preauthorize — requires
   status: enrolled.
4. POST /v1/banking/bank_payrolls/{id}/authorize — final step; the payroll
   ends approved or declined.

See Tutorial: Emit a payroll and reconcile it.