[v0l]

Snapshot support on Proxmox, libvirt, marketplace nodes and the dummy host.

• VmHostClient: list_snapshots, create_snapshot, rollback_snapshot, delete_snapshot
• Proxmox keeps a running VM running; libvirt shuts it down for the copy and restarts it (UEFI guests cannot be snapshotted live)
• Only the newest snapshot can be restored (Proxmox ZFS refuses anything else), 409 names the newer ones to delete
• Restore reapplies the current config and starts the VM; CheckVm is queued even when it fails
• Create, restore and delete run as a VmSnapshot worker job, serialised with spawn, reinstall and upgrades
• Up to 3 snapshots per VM, names 2 to 40 chars (pve-configid), validated on path params too
• Reinstall and disk resize delete snapshots first
• API: GET/POST /api/v1/vm/{id}/snapshots, DELETE /api/v1/vm/{id}/snapshots/{name}, PATCH /api/v1/vm/{id}/snapshots/{name}/restore
• VM history: snapshot_created, snapshot_restored, snapshot_deleted

Tested: workspace unit tests, wiremock tests for Proxmox, real libvirt/QEMU (snapshots_survive_a_full_cycle_on_real_qemu). Not tested against a real Proxmox node.