feat(api): Add LXC snapshot routes

[pparage]

Context

The proxmox_controller role supports LXC snapshots (snapshot_lxc_create, snapshot_lxc_delete, snapshot_lxc_revert, snapshot_lxc_list) but these are not exposed via API routes.

Requirements

• POST /v0/admin/proxmox/lxc/{id}/snapshot/create
• DELETE /v0/admin/proxmox/lxc/{id}/snapshot/delete
• POST /v0/admin/proxmox/lxc/{id}/snapshot/revert
• GET /v0/admin/proxmox/lxc/{id}/snapshot/list
• Pydantic schemas for each

Priority

Important. Should be implemented alongside B4 (LXC lifecycle routes).

References

• Gap analysis: gap-analysis.md issue B9