[feature]: Cluster backup and restore commands (Velero-inspired)

[devantler]

User Story

As a Kubernetes operator,
I want ksail cluster backup and ksail cluster restore commands,
So that I can backup and restore a cluster's Kubernetes resources and persistent volumes for disaster recovery or cluster migration.

Acceptance Criteria

Backup Command (ksail cluster backup)

• Creates a backup archive (.tar.gz) containing Kubernetes resources and volume data.
• Backs up all Kubernetes resources as YAML files organized by namespace (similar to Velero's tarball approach).
• Backs up persistent volume data using file system backup (copying data from PVCs).
• Includes a backup.yaml metadata file with: version, timestamp, cluster name, distribution, provider, and list of backed-up resources.
• Supports --include-volumes flag to back up persistent volume data (default: true).
• Backs up resources in a consistent order (CRDs → Namespaces → StorageClasses → PVs → PVCs → Secrets → ConfigMaps → Services → Deployments → etc.).

Restore Command (ksail cluster restore)

• Restores Kubernetes resources from a backup archive to a target cluster.
• Restores resources in the correct order (CRDs first, then namespaces, then storage resources, then workloads).
• Restores persistent volume data by provisioning new PVCs and copying data back.
• Supports --existing-resource-policy flag with options: none (skip existing, default) or update (patch existing resources).
• Labels restored resources with ksail.io/backup-name and ksail.io/restore-name for traceability.

Limitations (v1)

• No secret encryption in v1 (secrets exported as plain YAML - recommend using SOPS separately).
• No cloud provider volume snapshots in v1 (uses file system backup approach only).
• No Hetzner/block storage snapshots in v1.