_promote_stable_versions_for_note is the pure-function core: walks
versions chronologically and pins any with a >= AUTO_PIN_STABILITY_DAYS
(2-day) gap to the next version (or to now, for the latest). Auto-
generated label describes the stability window.
_scan_one_note loads versions for one note, runs the promotion, commits
mutations to the attached rows, then calls prune_auto_pins to cap the
auto bucket. scan_user_for_auto_pins fans out across the user's notes;
scan_all_users_for_auto_pins is the top-level entrypoint for the cron.
Per-note and per-user errors are caught and logged.
