Updates copy, folder, schedule, redirect, style, logo, and a curated subset of UI settings.
Style and branding can also be updated via PATCH /surveys/{survey_id}/styles.
Feature gates: upload_logo (logo fields), remove_branding (settings.hide_branding),
schedule_voiceform_end_date (ends_at), redirect (redirect_link).
settings is merged partially — only known keys are applied; other settings are preserved.