docker exec defaults to root, so the systemd base-backup timer connected to the
database as role "root" (then "postgres") — neither exists; the superuser role is
POSTGRES_USER (scrabble). Run the timer's pgBackRest as the postgres OS user (-u postgres,
for lock-dir/PGDATA consistency with archive-push) and connect with --pg1-user=scrabble.
archive_command (run by the postgres server process) was already correct.
Also record the point-in-time-recovery arming + restore drill in deploy/README.md
(correct the runbook commands to the same invocation, fill the drill log) and mark the
durability work done in PLAN.md.
Ilia Denisov