fix: switch to API token authentication for Proxmox

- Replace user/password auth with API token auth
- Update provider config to use pm_api_token_id and pm_api_token_secret
- Update workflow secrets to use PM_API_TOKEN_ID and PM_API_TOKEN_SECRET
- Remove unused pm_user and proxmox_password variables

terraform/variables.tf

@@ -1,5 +1,12 @@
-variable "proxmox_password" {
-  type = string
+variable "pm_api_token_id" {
+  type        = string
+  description = "Proxmox API token ID (format: user@realm!tokenid)"
+}
+
+variable "pm_api_token_secret" {
+  type        = string
+  sensitive   = true
+  description = "Proxmox API token secret"
 }
 
 variable "target_node" {
@@ -38,10 +45,6 @@ variable "pm_api_url" {
   type = string
 }
 
-variable "pm_user" {
-  type = string
-}
-
 variable "alpaca_vm_count" {
   type        = number
   default     = 1