Overview
Creates a new operator user and optionally assigns groups and store restrictions.

User scoping
Admin endpoint. Only operator users can call this endpoint (dj-client: OPERATOR).

Store scoping
dj-store and dj-store-view define the request context. The created operator user can be restricted to specific stores using stores.

Validation rules
Required fields: email, firstName, lastName, civility, locale.
If stores is provided, it must be an array of store external identifiers (externalId); duplicates are ignored.

Business rules
If stores is omitted or an empty array, the operator user is allowed on all stores (no restriction).

Errors
Returns validation errors (400), authentication errors (401), authorization errors (403), and unexpected server errors (500).