[ci skip] Migrate 22 platform service states to stacks/platform

State migration for all platform services from root state to
state/stacks/platform/terraform.tfstate. Key changes:
- module.kubernetes_cluster.module.X["key"] -> module.X
- Removed null_resource.core_services from root state
- Imported traefik helm_release (was missing from state)
- Fixed helm provider syntax (kubernetes = {} not kubernetes {})
- Added secrets symlink for TLS cert file() resolution
- Platform terragrunt plan: 0 add, 24 change (cosmetic drift), 0 destroy

stacks/platform/secrets

@@ -0,0 +1 @@
+../../secrets

terragrunt.hcl

@@ -47,7 +47,7 @@ provider "kubernetes" {
 }
 
 provider "helm" {
-  kubernetes {
+  kubernetes = {
     config_path = var.kube_config_path
   }
 }