Get one custom field definition by its numeric id, scoped to the authenticated company.

Use this to resolve a field's current type, options, and settings — for example before writing a value into an entity's custom_data, where this same id is the map key. Returns 404 when no field with that id belongs to the company (a field owned by another company is indistinguishable from one that does not exist). Read-only; touches nothing in inventory or compliance.

Required permission: settings_permissions_custom_fields.