feat(db): switch to discourse/postgres image (auto-upgrade)

Move the db off the bitnami-era pgvector:pg17 + hand-rolled pg_upgrade entrypoint
to discourse/postgres:pg18 (pgvector + discourse's auto-upgrade layer). The image
runs the in-place major-version pg_upgrade itself on boot; the recipe configures it
via env:

- a small inline entrypoint injects the db password secret into $DB_PASSWORD (the
  image expects it in the env, no *_FILE support)
- POSTGRES_USER (the install user pg_upgrade must match) defaults to 'postgres' --
  correct for fresh installs and bitnami-origin clusters -- overridable from .env
- POSTGRES_INITDB_ARGS=--no-data-checksums so the new pg18 cluster matches pre-18
  clusters (pg18 initdb enables checksums by default; pg_upgrade needs a match)

- mount postgresql_data at /var/lib/postgresql (versioned PGDATA .../18/docker)
- pg_backup.sh uses POSTGRES_USER for the dump/drop/recreate; fix paths
- document the POSTGRES_USER override in .env.sample, README and the release note
- drop entrypoint.postgres.sh.tmpl

Tested on cctest: pg17->pg18 upgrade preserves data and serves over HTTPS; fresh
install works; backup+restore round-trips.

.env.sample

@@ -21,3 +21,9 @@ DISCOURSE_DEVELOPER_EMAILS=admin@example.com
 #SECRET_SMTP_PASSWORD_VERSION=v1
 
 SECRET_DB_PASSWORD_VERSION=v1
+
+# Postgres bootstrap superuser (the cluster's "install user"). Defaults to
+# `postgres`, which matches fresh installs and bitnami-origin clusters. Only set
+# this if you are upgrading a cluster that was bootstrapped with a different
+# superuser (e.g. `discourse`) — a postgres major upgrade fails unless it matches.
+#POSTGRES_USER=postgres