[ci skip] Move Terraform modules into stack directories

Move all 88 service modules (66 individual + 22 platform) from
modules/kubernetes/<service>/ into their corresponding stack directories:

- Service stacks: stacks/<service>/module/
- Platform stack: stacks/platform/modules/<service>/

This collocates module source code with its Terragrunt definition.
Only shared utility modules remain in modules/kubernetes/:
ingress_factory, setup_tls_secret, dockerhub_secret, oauth-proxy.

All cross-references to shared modules updated to use correct
relative paths. Verified with terragrunt run --all -- plan:
0 adds, 0 destroys across all 68 stacks.

stacks/platform/modules/metrics-server/main.tf

@@ -0,0 +1,29 @@
+variable "tls_secret_name" {}
+variable "tier" { type = string }
+
+resource "kubernetes_namespace" "metrics-server" {
+  metadata {
+    name = "metrics-server"
+    labels = {
+      tier = var.tier
+    }
+  }
+}
+
+module "tls_secret" {
+  source          = "../../../../modules/kubernetes/setup_tls_secret"
+  namespace       = kubernetes_namespace.metrics-server.metadata[0].name
+  tls_secret_name = var.tls_secret_name
+}
+
+resource "helm_release" "metrics-server" {
+  namespace        = kubernetes_namespace.metrics-server.metadata[0].name
+  create_namespace = false
+  name             = "metrics-server"
+  atomic           = true
+
+  repository = "https://kubernetes-sigs.github.io/metrics-server/"
+  chart      = "metrics-server"
+
+  values = [templatefile("${path.module}/values.yaml", {})]
+}

stacks/platform/modules/metrics-server/values.yaml

@@ -0,0 +1,2 @@
+args:
+  - "--kubelet-insecure-tls"