Success Envelope
A successful response hasok: true and a data value.
data can be an object, array, string, number, boolean, or null, depending on the endpoint.
Error Envelope
An error response hasok: false and an error object.
details may be included for validation issues or structured domain context:
Status Codes
Common Error Codes
Exact domain error codes can vary by endpoint. Clients should branch on HTTP status first, then use
error.code for product-specific handling.

