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/App.tsx

@@ -272,6 +272,8 @@ function App() {
                         <Route path="accounting" element={<AccountingLayout />}>
                           <Route element={<RequireFeature flag="incomingInvoices" />}>
                             <Route path="inbox" element={<AccountingInboxPage />} />
+                          </Route>
+                          <Route element={<RequireFeature flag="expenses" />}>
                             <Route path="expenses" element={<ExpensesLedgerPage />} />
                           </Route>
                           <Route element={<RequireFeature flag="taxReport" />}>

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;
 };

frontend/src/contexts/FeatureFlagsContext.tsx

@@ -48,9 +48,12 @@ export const DEFAULT_FLAGS: FeatureFlags = {
   // Accounting (migration 122). Top-level MASTER for the Accounting
   // section (separate from CRM). Sub-features below require it.
   accounting: false,
-  // Incoming invoices (migration 124) — supplier-invoice capture +
-  // expenses + re-bill. Accounting sub-feature; requires `accounting`.
+  // Incoming invoices (migration 124) — external supplier-invoice capture +
+  // re-bill. Accounting sub-feature; requires `accounting`.
   incomingInvoices: false,
+  // Expenses (migration 127) — internal expenses (mileage / per-diem / cash).
+  // Separate Accounting sub-feature; requires `accounting`.
+  expenses: false,
 };
 
 export const FEATURE_FLAGS_QUERY_KEY = ['feature-flags'] as const;
@@ -87,6 +90,7 @@ function applyDependencyRules(flags: FeatureFlags): FeatureFlags {
   if (out.accounting === false) {
     out.taxReport = false;
     out.incomingInvoices = false;
+    out.expenses = false;
   }
   // Clients parent flag is DERIVED from its children. Admins don't
   // toggle it directly — enabling any CRM-area sub-feature

frontend/src/features/settings/tabs/FeaturesTab.tsx

@@ -18,6 +18,7 @@ import {
   Calculator,
   Landmark,
   ScanLine,
+  Wallet,
 } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
 import { Button, Card } from '../../../components/common';
@@ -331,6 +332,25 @@ export const FeaturesTab: React.FC = () => {
               'Enable Accounting first — Incoming invoices live in the Accounting section.',
             ) : undefined}
           />
+
+          <FeatureCard
+            icon={Wallet}
+            title={t('settings.features.expenses.title', 'Expenses')}
+            description={t(
+              'settings.features.expenses.description',
+              'Internal expenses (mileage, per-diem, cash) booked to an event or the company, with optional proof. Separate from incoming supplier invoices. Configure km / per-diem rates and the proof requirement in the Accounting settings tab.',
+            )}
+            status="new"
+            statusLabel={statusLabel('new')}
+            sidebarLabel={t('settings.features.expenses.sidebar', 'Expenses')}
+            enabled={staged.expenses}
+            onToggle={(next) => setFlag('expenses', next)}
+            disabled={!staged.accounting}
+            lockedReason={!staged.accounting ? t(
+              'settings.features.expenses.requiresAccounting',
+              'Enable Accounting first — Expenses live in the Accounting section.',
+            ) : undefined}
+          />
         </Section>
 
         {/* Insights & Access */}

frontend/src/i18n/locales/de.json

@@ -1650,6 +1650,12 @@
         "sidebar": "Eingang",
         "requiresAccounting": "Bitte zuerst Buchhaltung aktivieren — Eingangsrechnungen liegen im Buchhaltungsbereich."
       },
+      "expenses": {
+        "title": "Aufwände",
+        "description": "Interne Aufwände (Kilometer, Spesenpauschale, Barbelege), gebucht auf ein Event oder die Firma, mit optionalem Beleg. Getrennt von Eingangsrechnungen.",
+        "sidebar": "Aufwände",
+        "requiresAccounting": "Bitte zuerst Buchhaltung aktivieren — Aufwände liegen im Buchhaltungsbereich."
+      },
       "analytics": {
         "title": "Statistiken",
         "description": "Speichernutzung, Galerie-Aufrufe, Download-Zahlen und Statistiken pro Veranstaltung."

frontend/src/i18n/locales/en.json

@@ -1208,6 +1208,12 @@
         "sidebar": "Incoming",
         "requiresAccounting": "Enable Accounting first — Incoming invoices live in the Accounting section."
       },
+      "expenses": {
+        "title": "Expenses",
+        "description": "Internal expenses (mileage, per-diem, cash) booked to an event or the company, with optional proof. Separate from incoming supplier invoices.",
+        "sidebar": "Expenses",
+        "requiresAccounting": "Enable Accounting first — Expenses live in the Accounting section."
+      },
       "analytics": {
         "title": "Analytics",
         "description": "Storage usage, gallery views, download counts, and per-event stats."

frontend/src/services/featureFlags.service.ts

@@ -46,9 +46,12 @@ export type FeatureKey =
   // section (separate from CRM). Its sub-features (tax export, incoming
   // invoices) require it. Strictly opt-in.
   | 'accounting'
-  // Incoming invoices (migration 124) — supplier-invoice capture +
-  // expenses + re-bill. Accounting sub-feature; requires `accounting`.
-  | 'incomingInvoices';
+  // Incoming invoices (migration 124) — external supplier-invoice capture +
+  // re-bill. Accounting sub-feature; requires `accounting`.
+  | 'incomingInvoices'
+  // Expenses (migration 127) — internal expenses (mileage / per-diem / cash).
+  // Separate Accounting sub-feature; requires `accounting`.
+  | 'expenses';
 
 export type FeatureFlags = Record<FeatureKey, boolean>;