extract remaining 19 modules from platform, complete stack split [ci skip]

Phase 3: all 27 platform modules now run as independent stacks.
Platform reduced to empty shell (outputs only) for backward compat
with 72 app stacks that declare dependency "platform".
Fixed technitium cross-module dashboard reference by copying file.
Woodpecker pipeline applies all 27+1 stacks in parallel via loop.
All applied with zero destroys.

stacks/headscale/main.tf

@@ -0,0 +1,21 @@
+variable "tls_secret_name" { type = string }
+variable "nfs_server" { type = string }
+
+data "vault_kv_secret_v2" "secrets" {
+  mount = "secret"
+  name  = "platform"
+}
+
+locals {
+  homepage_credentials = jsondecode(data.vault_kv_secret_v2.secrets.data["homepage_credentials"])
+}
+
+module "headscale" {
+  source           = "./modules/headscale"
+  tls_secret_name  = var.tls_secret_name
+  nfs_server       = var.nfs_server
+  headscale_config = data.vault_kv_secret_v2.secrets.data["headscale_config"]
+  headscale_acl    = data.vault_kv_secret_v2.secrets.data["headscale_acl"]
+  homepage_token   = try(local.homepage_credentials["headscale"]["api_key"], "")
+  tier             = local.tiers.core
+}