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

Replace the bitnami-era pgvector:pg17 db + hand-rolled pg_upgrade entrypoint
with discourse/postgres:pg18 (pgvector + discourse's auto-upgrade layer, as
suggested on coop-cloud/discourse#16). The image does the in-place major-version
pg_upgrade itself on boot, so the recipe just configures it via env:

- cc-db-entrypoint.sh is now a 5-line wrapper that only injects the password
  secret (the image reads DB_PASSWORD/POSTGRES_PASSWORD from env, no *_FILE);
- POSTGRES_USER (the install user pg_upgrade must match) defaults to the image's
  'postgres' -- correct for fresh installs and bitnami-origin clusters -- and is
  overridable from the app .env for a cluster bootstrapped with another superuser;
- POSTGRES_INITDB_ARGS=--no-data-checksums so the new pg18 cluster matches the
  pre-18 clusters (pg18 initdb enables checksums by default, pg_upgrade requires
  a match). No need to install the old postgres in our wrapper anymore.

- mount postgresql_data at /var/lib/postgresql (versioned PGDATA .../18/docker)
- pg_backup.sh: detect the superuser at runtime; fix paths for the new layout
- document POSTGRES_USER override in .env.sample and README
- bump DB_ENTRYPOINT_VERSION v7, PG_BACKUP_VERSION v3 (immutable swarm configs)
- drop entrypoint.postgres.sh.tmpl

Verified on cctest: pg17->pg18 upgrade (install user 'postgres', checksums off)
preserves data and serves over HTTPS; fresh install also works.

cc-db-entrypoint.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Co-op Cloud entrypoint wrapper for the discourse/postgres image.
+#
+# The image (https://github.com/discourse/discourse-postgres) handles everything
+# itself, including the in-place major-version pg_upgrade on boot. The only thing
+# it can't do for us is read the docker secret: it expects the DB password in the
+# DB_PASSWORD / POSTGRES_PASSWORD env vars (no *_FILE support), so inject it from
+# /run/secrets here, then hand off to the image's real entrypoint.
+#
+# The install user and data-checksum settings the upgrade needs are passed as
+# plain env vars in compose.yml (POSTGRES_USER, POSTGRES_INITDB_ARGS).
+set -e
+
+if [ -f /run/secrets/db_password ]; then
+  DB_PASSWORD="$(cat /run/secrets/db_password)"
+  export DB_PASSWORD
+  export POSTGRES_PASSWORD="$DB_PASSWORD"
+fi
+
+exec run-postgres.sh postgres