Server-authoritative recognition for one AP item

DARK — 404s unless company_subledger_flags.ar_ap_items_enabled is true. Returns { treatment, monthlyCents, recognizedToDateCents, remainingCents, totalCents, schedule[{ month, amountCents, posted }] }. Treatment and the monthly split are derived from the item's service period; posted flags and recognized-to-date come strictly from the ledger (posted amortization runs, else the item's ledger-posted authority) — never client-computed.