Create an invoice from the composer (server-owned totals)

DARK — 404s unless ar_ap_items_enabled. Line items carry revenue category, quantity, unit price, tax bps, and a per-line service period; every line subtotal/tax/total and the item amount_cents are recomputed server-side (client totals are rejected by the strict schema). dueDate derives from terms when not supplied; status "sent" composes and sends in one call.