Registers a new client company under the partner and returns a company-scoped API key. Endpoint: POST /v1/onboard (Partner Key) 
Partner ──POST /v1/onboard──> API ──> Validate Partner is Active
                                  ──> Create Company + default BusinessEntity
                                  ──> Generate Company API Key
       <── 200 OK ──────────── API
           { companyId, businessEntityId, companyApiKey, message }
Request: 
{
  "name": "Acme Corporation",
  "legalName": "Acme Corp Ltd.",
  "country": "US",
  "taxId": "123-456-789",
  "phone": "+1234567890",
  "address": "123 Main St",
  "postalCode": "10001",
  "city": "New York"
}
Response (200): 
{
  "companyId": "01HXYZ...",
  "businessEntityId": "01HXYZ...",
  "companyApiKey": "eve_ck_...",
  "message": "Company onboarded successfully"
}
Key behaviors:
  • companyApiKey is returned in plain text only once — store it securely
  • A default BusinessEntity is auto-created and pre-approved
  • Use the Compliance Framework and Control discovery endpoints to find framework/control IDs for document submission