The Postgres major-version upgrade and ClickHouse changes are now handled
automatically (pgautoupgrade + resilient entrypoint), so this is a
backward-compatible minor bump, not a major one.
Replace the hand-rolled entrypoint.postgres.sh.tmpl (which apt-installed
the old PG binaries and ran initdb + pg_upgrade --link by hand) with the
pgautoupgrade/pgautoupgrade:18-alpine image, matching the other recipes.
PGDATA is pinned to the legacy /var/lib/postgresql/data so the existing
cluster on the volume is upgraded in place rather than re-initialised.
Drops the db_entrypoint config and DB_ENTRYPOINT_VERSION.
Make the clickhouse-backup install REQUIRED: if it cannot be fetched
after all retries the entrypoint aborts (non-zero exit, set -e) and
clickhouse-server is not started, so the deploy fails loudly rather than
coming up without backup/restore capability.
