Fix: TypeScript-Fehler im multer Upload-Endpoint

- @types/multer zu devDependencies hinzugefügt (fehlte → TS7016)
- Explizite Callback-Typen in diskStorage/fileFilter (TS7006)
- Express.Multer.File durch lokalen MulterFile-Type ersetzt (TS2694)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/package.json

@@ -24,6 +24,7 @@
   "devDependencies": {
     "@types/cors": "^2.8.17",
     "@types/express": "^4.17.21",
+    "@types/multer": "^1.4.12",
     "@types/node": "^20.12.12",
     "ts-node": "^10.9.2",
     "typescript": "^5.5.4"