feat(api): Add template management routes

[pparage]

Context

The proxmox_controller role supports template operations (template_create, template_convert_vm_to_template, template_cloudinit_import_disk, cloudinit_set_variables) but the API only exposes listing templates via storage routes — no creation/conversion.

Requirements

• POST /v0/admin/proxmox/templates/create → template_create
• POST /v0/admin/proxmox/templates/convert → template_convert_vm_to_template
• POST /v0/admin/proxmox/templates/cloudinit/import-disk → template_cloudinit_import_disk
• POST /v0/admin/proxmox/templates/cloudinit/set-variables → cloudinit_set_variables

Priority

Nice-to-have.

References

• Gap analysis: gap-analysis.md issue B18