Register tenant

Creates the user and an owner-scoped tenant in a single transaction. The tenant display name mirrors `fullName`; the URL slug is derived from it server-side and a random suffix is appended on collision (the user never picks one). Owners can rename the tenant later from the settings.

curl -X POST "https://api.monitoring.crahe-arthur.com/api/v1/auth/register" \  -H "Content-Type: application/json" \  -d '{    "fullName": "Arthur Crahé",    "email": "admin@acme.example",    "password": "correct horse battery staple",    "plan": "free"  }'

{
  "accessToken": "eyJhbGciOi...",
  "user": {
    "id": "018f7b9a-57dd-4748-9e13-6e3e3e5b9eaf",
    "email": "admin@acme.example",
    "fullName": "Arthur Crahé"
  },
  "tenant": {
    "id": "018f7b9a-6f8d-4c1d-8d72-1bbecdadc101",
    "slug": "acme-monitoring",
    "orgRole": "owner"
  }
}