Validates patch via Zod, deep-merges with current settings in
a transaction, persists to DB, and emits pg_notify on
scrum4me_changes for cross-tab/cross-device sync. Demo accounts
get 403, unauthenticated 401, invalid input 422.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
