feat: replace per-node baked ISOs with HTTP answer server

Replace 4 per-node auto-install ISOs (~4GB) with 2 generic ISOs
(~2GB) served by an HTTP answer server that routes per-node configs
by MAC address.

New flow:
1. Deterministic MAC addresses assigned per node (AA:BB:CC:00:VV:NN)
2. Per-MAC answer.toml files generated in answers/ directory
3. HTTP answer server (slothcroissant/proxmox-auto-installer-server)
   managed by Terraform, serves answer files on port 8000
4. Generic ISOs prepared with --fetch-from http --url pointing to
   the answer server
5. PVE installer POSTs system info, server matches MAC to answer file

Benefits:
- 50% reduction in ISO disk/tmp usage (2 ISOs instead of 4)
- Faster ISO preparation (2 builds instead of 4)
- Answer files can be updated without rebuilding ISOs
- First-boot script embedded in generic ISO via --on-first-boot

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/infrastructure/variables.tf

@@ -26,6 +26,7 @@ variable "pve_instances" {
     iso_local_path = string
     vm_id          = number
     vm_name        = string
+    mac_address    = string
   }))
 }
 
@@ -88,3 +89,13 @@ variable "docker_host_ip" {
   type        = string
 }
 
+variable "answer_files_dir" {
+  description = "Host path to the directory containing per-MAC answer.toml files"
+  type        = string
+}
+
+variable "default_answer_file" {
+  description = "Host path to the default answer.toml file"
+  type        = string
+}
+