feat(billing): filter, show and export invoice payment method

The settled method is split across two stores: a gateway settlement records
the real provider on the linked freight.payments row (cbe-bill, telebirr)
while the invoice's own payments ledger only writes a flat "GATEWAY"; a
manual settlement has no payments row at all and the ledger is the only
source (BANK_TRANSFER, OFFLINE, or whatever PayInvoiceDto.method carried).

invoicePaymentMethodExpr folds both into one UPPER_SNAKE vocabulary —
provider first, newest ledger entry as the fallback — and the list filter,
the export field and the export filter all use that same expression, so the
screen and the file can never disagree.

The paymentMethods param is deliberately not validated against a fixed list:
the manual pay endpoint takes a free-form method, so an IsIn would silently
drop real values.

apps/edr-freight-api/src/modules/billing/invoice-settlement.util.ts

@@ -34,3 +34,43 @@ export function applySettlement(
   const balanceAmount = Math.max(0, round2(total - paidAmount));
   return { paidAmount, balanceAmount, fullyPaid: paidAmount >= total };
 }
+
+/**
+ * SQL for an invoice's settled payment method, normalised to one vocabulary.
+ *
+ * Two sources have to be merged: gateway settlements carry the real provider on
+ * the linked `freight.payments` row (`cbe-bill`, `telebirr`, …) while the
+ * invoice's own `payments` ledger only records a flat `"GATEWAY"`; manual
+ * settlements have no payments row at all and the ledger is the ONLY source
+ * (`BANK_TRANSFER`, `OFFLINE`, or whatever `PayInvoiceDto.method` carried).
+ * So: provider first, newest ledger entry as the fallback.
+ *
+ * `-> -1` is the last ledger element — the ledger is appended newest-last.
+ * `::text` is not cosmetic: `payments.method` is a real Postgres enum, and
+ * COALESCE against a text fallback fails without the cast.
+ *
+ * Normalised UPPER_SNAKE so `cbe-bill` and a hand-typed `CBE_BILL` are one
+ * value on screen, in the filter and in the export.
+ */
+export const invoicePaymentMethodExpr = (invoice: string, payment: string): string =>
+  `UPPER(REPLACE(COALESCE(${payment}.method::text, ${invoice}.payments -> -1 ->> 'method'), '-', '_'))`;
+
+/**
+ * The methods the filter offers. Not exhaustive by construction — the manual
+ * pay endpoint takes a free-form `method` string — so nothing validates against
+ * this list; it is the pick-list, not a constraint.
+ */
+export const INVOICE_PAYMENT_METHODS = [
+  "TELEBIRR",
+  "CBE_BIRR",
+  "CBE_BILL",
+  "EBIRR",
+  "WAAFI",
+  "CARD",
+  "DMONEY",
+  "CAC_BANK",
+  "BANK_TRANSFER",
+  "OFFLINE",
+  /** Settled at a gateway whose provider row is no longer linked. */
+  "GATEWAY",
+] as const;