POST /v1/auth/register：PIN 用户注册

curl -X POST http://localhost:8001/v1/auth/register \
  -H "Content-Type: application/json" \
  -d '{"username":"alice","email":"[email protected]","password":"secret123"}'

{
  "id": "usr_2vPqN9xL5wR8"
}

{
  "code": 422,
  "data": null,
  "message": "validation failed",
  "detail": "username already exists"
}