### Schritt 1: Gruppen IDs herausfinden (als Admin)
GET http://localhost:8080/api/admin/groups
Authorization: Basic admin admin123

### Schritt 2: Einen neuen User "Onkel_Heinz" anlegen
POST http://localhost:8080/api/admin/users
Authorization: Basic admin admin123
Content-Type: application/json

{
  "username": "Onkel_Heinz",
  "email": "heinz@familie.de",
  "initialPassword": "geheim",
  "groupIds": [
    "26f33481-b68a-410d-852b-a081ca9ae930"
  ]
}