Generate the initial payroll data for employees and contractors in a selected pay period. This is the first step of the payroll process.

Flow: GET /payrolls/period-options → POST /payrolls/initialize → edit payrollData → POST /payrolls (create draft) → review → approve.

Each employee/contractor entry contains payment_method and note at the root, plus either:

• payrollData (with time_totals and jobCostingData) — for employees with no mid-period rate changes, OR
• splits — an array of sub-periods, each with its own payrollData, time_totals, and jobCostingData, for employees who had a pay rate change during this period.

payrollData and splits are mutually exclusive — an entry will never contain both. When splits is present, each split's payrollData is the source of truth.

The response also includes actions.create_draft — a pre-built request body for POST /payrolls that the client can modify (adjust hours, amounts, notes) and submit directly to create a draft.

Supports regular payroll initialization via pay schedule group and off-cycle initialization via explicit dates. Requires the payroll plan or higher.