Read monthly measured usage by project and meter

Owner-only event-month ledger totals, including signed corrections posted by as_of. Returns up to 20 projects per page in ID order; use next_cursor with the same month. Current unresolved reservations are separate from measured consumption. Null environment_id means project-shared cost, never guessed Dev allocation. Only posted measurements are included; this is not a complete provider invoice or zero-usage guarantee. Billing may arrive later. No credit is granted or charged by this read; it remains usable at zero credits.