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/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