Add a restic VM-data backup guide for the tenancy/vm module

Description

The tenancy/vm examples document how to provision a standalone Harvester VM, but not how to back up its data. Product teams own their VM-data backups to their own S3 buckets and currently have no guide for it.

This is the general-VM counterpart to the cluster backups guide (#218 / #219, etcd + Velero): that covers RKE2 clusters; this covers plain VMs.

Proposal

Add modules/tenancy/vm/examples/BACKUPS.md — a self-service restic guide, matching the style of the existing k8s-cluster/examples/BACKUPS.md:

• Bucket + least-privilege IAM user (policy JSON including multipart actions) plus a repository password.
• The in-guest install: cloud-init (base64-embedded scripts) that installs restic and runs a systemd backup timer, passed via the vm module's user_data.
• Verify, and restore — into a fresh VM (same DC), a single file / subtree (file-level), and cross-DC DR.
• A scheduling/retention reference and a production-hardening checklist.