fix: use TF_VAR_ prefix for token credentials

- Restore pm_api_token_id and pm_api_token_secret variables
- Use TF_VAR_pm_api_token_id and TF_VAR_pm_api_token_secret env vars
- This is the standard Terraform way to pass variables via environment

terraform/variables.tf

@@ -1,3 +1,14 @@
+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" {
   type = string
 }