Forward expense recognition by month across posted AP items

DARK — 404s unless company_subledger_flags.ar_ap_items_enabled is true. Server-authoritative aggregation of the company's posted AP recognition by month; unposted intake is excluded (not a booked liability). Returns [{ offset, label, amountCents }] where offset 0 is the current month, negatives are past, positives are scheduled. Window defaults to 1 month back / 6 ahead (monthsBack / monthsAhead, each 0..24).