docs: add Proxmox setup guide for Terraform (#2)

## Summary

Add Proxmox setup documentation for Terraform with dedicated role and minimal privileges.

## Changes

- Add `docs/proxmox-setup.md` with complete setup guide
- Dedicated `TerraformRole` (no Administrator role needed)
- Both Web UI and CLI procedures
- SSH configuration for bpg/proxmox provider

## Privileges (Proxmox 9.1)

| Category  | Privileges |
|-----------|------------|
| Datastore | `Allocate`, `AllocateSpace`, `AllocateTemplate`, `Audit` |
| System    | `Audit`, `Console`, `Modify` |
| VM        | `Allocate`, `Audit`, `Clone`, `Config.*`, `Console`, `Migrate`, `PowerMgmt`, `Snapshot`, `Snapshot.Rollback`, `GuestAgent.Audit`, `GuestAgent.FileRead` |
| Pool      | `Allocate`, `Audit` |
| SDN       | `Use` |

> ⚠️ `VM.Monitor` deprecated in Proxmox 9 — use `VM.GuestAgent.Audit` and `VM.GuestAgent.FileRead` instead.

## Related

Closes #1

Co-authored-by: darnodo <sepales.pret0h@icloud.com>
Reviewed-on: #2

terraform/prod/variables.tf

@@ -59,7 +59,7 @@ variable "virtual_machines" {
     disk_format    = optional(string, "raw")
     network_bridge = optional(string, "vmbr0")
     network_model  = optional(string, "virtio")
-    username       = optional(string)
+    username       = optional(string, "damien")
     on_boot        = optional(bool, true)
     tags           = optional(list(string), [])