feat(accounting): split Incoming invoices vs Expenses - flags, schema, settings (stage 1)

Foundation for separating external supplier invoices from internal expenses,
per design review. This stage is additive + buildable; the service/route/UI
data rework follows in stage 2.

- Migration 126: incoming invoices own their payable on inbound_documents
  (supplier_paid/at/method/ref + disposition + tax_treatment + booking event_id
  + category_id + re-bill markup/linkage); expenses gain kind (amount/mileage/
  per_diem) + quantity + snapshotted rate_minor. Additive, hasColumn-guarded.
- Migration 127: seed `expenses` feature flag (default off) + accounting
  app_settings (accounting_km_rate_minor=70, accounting_per_diem_rate_minor=0,
  accounting_require_proof=false).
- Backend: `expenses` added to feature-flag known/defaults/dependency (forced
  off when the accounting master is off); new PUT /admin/settings/accounting
  (read via the generic GET /:type).
- Frontend: `expenses` flag (type + context + dependency); Features tab gets an
  Expenses sub-card; the Expenses sub-nav + route now gate on `expenses` (not
  incomingInvoices); AccountingIndex prefers inbox -> expenses -> tax.
- i18n: settings.features.expenses.* (EN + DE).

Verified: node -c; migration 124->126->127 harness (new columns, flag, settings
+ idempotency); en/de JSON valid; npm run build green.

frontend/src/components/admin/AccountingLayout.tsx

@@ -41,7 +41,7 @@ export const AccountingLayout: React.FC = () => {
       to: '/admin/accounting/expenses',
       label: t('accounting.subnav.expenses', 'Expenses'),
       icon: Wallet,
-      featureFlag: 'incomingInvoices',
+      featureFlag: 'expenses',
     },
     {
       key: 'tax-report',
@@ -159,6 +159,7 @@ export const AccountingLayout: React.FC = () => {
 export const AccountingIndex: React.FC = () => {
   const { flags } = useFeatureFlags();
   if (flags.incomingInvoices) return <Navigate to="/admin/accounting/inbox" replace />;
+  if (flags.expenses) return <Navigate to="/admin/accounting/expenses" replace />;
   if (flags.taxReport) return <Navigate to="/admin/accounting/tax-report" replace />;
   return null;
 };