Postgres operator backup configuration

The Zalando Postgres operator schedules pg_dumpall with a Kubernetes CronJob. Backups write to the postgres bucket on the cluster's Minio instance.

The longhorn-minio-credentials ExternalSecret supplies credentials. Longhorn uses this secret too, so one Minio user handles every backup. It must also expose LOGICAL_BACKUP_S3_ENDPOINT so the job points at Minio.

Set logical_backup_s3_endpoint in values.yaml to the Minio S3 endpoint URL (the value of LOGICAL_BACKUP_S3_ENDPOINT exposed by the longhorn-minio-credentials secret). The default schedule stores a dump every night at 03:00 Coordinated Universal Time. Adjust logical_backup_schedule if needed.