terraform: use local storage for cloud-init disks

Fix error "500 can't upload to storage type 'zfspool'" by using "local"
storage pool for cloud-init disks instead of the VM's storage pool.

Cloud-init disks require storage that supports ISO/snippet content types,
which zfspool does not. The "local" storage pool (directory-based) supports
this content type.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

terraform/cloud-init.tf

@@ -8,7 +8,7 @@ resource "proxmox_cloud_init_disk" "ci" {
 
   name     = each.key
   pve_node = each.value.target_node
-  storage  = each.value.storage
+  storage  = "local"  # Cloud-init disks must be on storage that supports ISO/snippets
 
   # User data includes SSH keys and optionally NIXOS_FLAKE_BRANCH
   user_data = <<-EOT