Forward-port of the release/5.1 fix (2c46c6c2d).
UpdateAlertConfig used to log SaveAlertConfig failures and still tell
the client "saved successfully", leaving the in-memory state with the
new override but the on-disk file untouched. On the next config reload
or process restart, the override silently vanished and the user saw
their threshold "revert" with no surfaced error. Return HTTP 500 with
the persistence error so the frontend can show a real save-failed
toast instead of false confidence.
rcourtman
80aab47da6