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.

release/1.0.0+3.5.3

@@ -8,3 +8,12 @@ Rename these in your app's .env (the values carry over):
   DISCOURSE_SMTP_USER     --> DISCOURSE_SMTP_USER_NAME
   DISCOURSE_SMTP_AUTH     --> DISCOURSE_SMTP_AUTHENTICATION
   DISCOURSE_SMTP_PROTOCOL --> DISCOURSE_SMTP_ENABLE_START_TLS (takes a boolean true/false, not the old tls/ssl value, so translate it rather than copying it straight across)
+
+WARNING: if your deployment's database has an "install user" other than `postgres`
+(some older deployments do), you must set the POSTGRES_USER env var in your .env
+for this migration, otherwise the postgres upgrade aborts with an install-user
+mismatch.
+
+Check your old deployment's install user before upgrading (if this command returns postgres, then you do not need to set this env):
+
+  abra app run YOURAPPDOMAIN db -- psql -U discourse -tAc 'select rolname from pg_roles where oid = 10'