feat(billing): search invoices by PNR and transaction ref

Both are numbers a customer or a provider support desk quotes back, so they
belong in the free-text box rather than behind a filter pill.

The transaction id and merchant order id are plain ORs — the payment alias is
already joined by every caller of applyInvoiceFilters. The PNR folds into the
existing booking EXISTS block instead of adding a second subquery, so it
inherits that block's correlation and also matches warehouse-, first-mile-
and last-mile-sourced invoices, not just booking-sourced ones.

bk is promoted to alwaysJoin now that the export's scope() references it.

apps/edr-freight-web/backoffice/src/pages/invoices/InvoicesPage.tsx

@@ -370,7 +370,7 @@ export default function InvoicesPanel() {
             <FilterBar
               defs={INVOICE_FILTER_DEFS}
               controls={controls}
-              searchPlaceholder="Search invoice, customer, booking ref, GRN or shipping line…"
+              searchPlaceholder="Search invoice, customer, booking ref, PNR, transaction ref, GRN or shipping line…"
               sortOptions={SORT_OPTIONS}
               viewId="invoices"
             >