store: Rebuild invalid postponed indexes left by interrupted runs

A CREATE INDEX CONCURRENTLY that is interrupted (e.g. by a node restart)
leaves an invalid index behind. Because postponed indexes are created with
'if not exists', such a leftover was skipped on retry and never rebuilt,
silently leaving the deployment without that index.

Before creating each postponed index, drop any invalid remnant so the
retry rebuilds it, mirroring what create_manual_index already does.