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/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."