Coolify provisions each Postgres resource with a random POSTGRES_USER; the
role "postgres" does not exist, so the hardcoded `psql -U postgres` fails
with FATAL: role "postgres" does not exist. Surfaced by the Task 9 restore
drill against box B. Expand $POSTGRES_USER inside the container (single-quoted
sh -c) so the restore uses the resource's own superuser.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>