chore: aktuellen Live-Stand sichern (Admin-API, Services, Auth-Erweiterungen)

Sichert den bisher uncommitteten Produktionsstand des Kundenbereich-Servers,
damit kuenftige Deploys (git reset --hard) nichts mehr verwerfen.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/routes/admin/invoiceClients.js

@@ -0,0 +1,19 @@
+import { Router } from 'express'
+import { requireAppwriteStaff } from '../../middleware/appwriteStaff.js'
+import { listInvoiceNinjaClients } from '../../services/invoiceNinja.js'
+
+const router = Router()
+
+router.use(requireAppwriteStaff)
+
+router.get('/', async (_req, res) => {
+  try {
+    const clients = await listInvoiceNinjaClients()
+    return res.json({ clients })
+  } catch (err) {
+    const status = err.status || 500
+    return res.status(status).json({ error: err.message || 'Kunden konnten nicht geladen werden' })
+  }
+})
+
+export default router