`missed_message_backfill.channels: []` (YAML list or JSON-list string) returned
an empty set on main, and the backfill logged "no channels configured" and
skipped. The gate-CSV refactor treated an empty result as "unset" and fell
through to the allowed-union-free-response default, scanning channels the
operator had explicitly disabled. An explicit list is now authoritative even
when empty; only the default "" string still falls through to env/default.
1d6fbd0d1f