Add PD to Serenity backup automation scaffolding

automation/bin/pd_backup_postgres.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/bash
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+# shellcheck disable=SC1091
+source "$SCRIPT_DIR/pd_backup_lib.sh"
+
+stamp="$(timestamp_utc)"
+dbs="${POSTGRES_DB_LIST:-}"
+
+if [[ -z "$dbs" ]]; then
+  echo "POSTGRES_DB_LIST is empty; nothing to dump." >&2
+  exit 1
+fi
+
+for db in $dbs; do
+  out="$PD_DB_DUMP_ROOT/${db}_${stamp}.sql.gz"
+  echo "Dumping Postgres database '$db' to $out"
+  docker_cmd exec "$POSTGRES_CONTAINER" pg_dump -U postgres "$db" | gzip -9 > "$out"
+done
+
+echo "Postgres dumps complete: $PD_DB_DUMP_ROOT"