_drain_webui_process_notifications now drops (consumes, never requeues) a
completion whose completed_at is older than a configurable cap
(HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS, default 6h; 0 disables).
Fail-open: events without completed_at (legacy agent) are never dropped.
Additive (+192/-0). Closes#4029.
Co-authored-by: allenliang2022
Co-authored-by: nesquena-hermes <agent@nesquena-hermes>