Add AWS Backup cost optimization rules

[dannysteenman]

What

Rules for backup sprawl: backup plans without cold-storage lifecycle transitions, aged/orphaned recovery points, and backup plans duplicating RDS automated backups.

Why

Backup storage grows forever by default. Warm backup storage costs ~10x cold storage, recovery points for deleted resources accumulate indefinitely, and it's common to find AWS Backup snapshotting RDS instances that already have automated backups — paying twice for the same protection.

Details

1. Backup plan rule with retention ≥ 90 days but no MoveToColdStorageAfterDays transition — IaC + discovery; Terraform aws_backup_plan lifecycle / CloudFormation AWS::Backup::BackupPlan.
2. Recovery points older than a review threshold, and recovery points whose source resource no longer exists — discovery; same shape as CLDBRN-AWS-EBS-7 / RDS-10 snapshot-age rules.
3. Backup plan selecting RDS resources whose instances already retain automated backups beyond the same window — discovery; review severity.

• New module packages/rules/src/aws/backup/, IDs CLDBRN-AWS-BACKUP-N; Resource Explorer mapping + vault/recovery-point hydration datasets per the provider-resource guide.