Import open invoices as opening balances

Import unpaid invoices from a raw QBO Open Invoices / A/R Aging Detail CSV (csvContent) or pre-mapped rows. NOTHING POSTS to the ledger: each invoice becomes an opening-balance detail row (status sent, openingBalance true, no journal entry) carrying its open balance — the A/R itself arrives with the company's GL/opening balances. Invoice numbers are kept as-is. Customer-capable contacts must already exist for every new invoice (manual customers count); if any required customer is missing, the whole import is refused with HTTP 409 and code open_invoice_customers_missing before any mutation. Re-running skips already-imported numbers without requiring those old customers again. Response includes importedBalanceCents to compare against the Accounts Receivable account.