Replace custom job field definitions

Whole-array replace of the board's custom job field definitions (jobFormConfig.customFields only: other job-form keys are untouched). Array order is display order. Empty array removes all fields. Content validation rejects empty labels, duplicate keys, optionless selects, inverted number bounds, and cap breaches (400). Changing type on an existing key is rejected (422 settings_custom_field_type_immutable); delete the field and recreate it. Renaming a key is delete+add and orphans per-job values under the old key. A field sent without visibility keeps its current visibility. The key of a deleted private field cannot be used for a new field (400), so its saved answers stay private. Response shape matches GET /v1/settings/job-form.