feat(db): switch to discourse/postgres image (auto-upgrade, no custom entrypoint file)

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 runs the in-place major-version
pg_upgrade itself on boot, so the recipe just configures it via env:

- the db password secret is read into $DB_PASSWORD by a small inline entrypoint
  (the image expects it in the env, no *_FILE support; the base image's
  POSTGRES_PASSWORD_FILE can't be used because run-postgres.sh pre-generates
  POSTGRES_PASSWORD). No separate entrypoint file/config any more.
- 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 needs a
  match).

- 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 PG_BACKUP_VERSION v3; drop DB_ENTRYPOINT_VERSION + 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.

README.md

@@ -43,6 +43,22 @@ override) so it works behind the reverse proxy.
 abra app run YOURAPPDOMAIN app discourse admin create
 ```
 
+## Postgres major version upgrades
+
+Handled automatically by the [`discourse/postgres`] image (pgvector + an
+auto-upgrade layer). On deploy it finds an older cluster, installs the old
+binaries and runs `pg_upgrade` into the new versioned data directory. No manual
+dump/restore needed.
+
+`pg_upgrade` must run as the old cluster's bootstrap superuser (its "install
+user"). The recipe uses `POSTGRES_USER`, which defaults to `postgres` — the right
+value for fresh installs and for clusters that came from the old bitnami recipe.
+If your cluster was bootstrapped with a different superuser (e.g. `discourse`),
+set `POSTGRES_USER` in the app `.env` before upgrading, otherwise `pg_upgrade`
+will refuse with an install-user mismatch.
+
+[`discourse/postgres`]: https://github.com/discourse/discourse-postgres
+
 ## Migrating from the previous (bitnami) recipe
 
 The official image stores uploads under `/shared` rather than bitnami's