feat(db): use POSTGRES_USER in pg_backup; document it in release note

- pg_backup.sh: use the db service's POSTGRES_USER (default postgres) for the
  dump/drop/recreate instead of detecting the superuser at runtime, since the
  recipe now sets that env var; one-line the constants comment
- release note: explain the in-place pg_upgrade + the POSTGRES_USER override
- bump PG_BACKUP_VERSION v4

Verified on cctest: backup + restore via the hooks round-trips with POSTGRES_USER.

pg_backup.sh

@@ -4,23 +4,13 @@
 
 set -e
 
-# constants 
+# dump goes at the volume root so backupbot's backup.sql label finds it
 BACKUP_FILE='/var/lib/postgresql/backup.sql'
 DATADIR="${PGDATA:-/var/lib/postgresql/18/docker}"
 DB_NAME="${POSTGRES_DB:-discourse}"
 
-# The bootstrap superuser (install user, oid 10) differs between deployments
-# (`postgres` on bitnami-origin clusters, `discourse` on others). Detect it at
-# runtime over the local trust socket rather than hard-coding a name.
-detect_superuser() {
-  local u name
-  for u in discourse postgres; do
-    name="$(psql -U "$u" -d "$DB_NAME" -tAc 'select rolname from pg_roles where oid = 10' 2>/dev/null | tr -d '[:space:]')"
-    if [ -n "$name" ]; then echo "$name"; return 0; fi
-  done
-  echo postgres
-}
-SU="$(detect_superuser)"
+# bootstrap superuser for the dump/drop/recreate; same POSTGRES_USER the db service sets
+SU="${POSTGRES_USER:-postgres}"
 
 function backup {
   pg_dump -U "$SU" "$DB_NAME" | gzip > "$BACKUP_FILE"