[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/echo/module/main.tf

@@ -0,0 +1,84 @@
+variable "tls_secret_name" {}
+variable "tier" { type = string }
+
+resource "kubernetes_namespace" "echo" {
+  metadata {
+    name = "echo"
+    labels = {
+      "istio-injection" : "disabled"
+      tier = var.tier
+    }
+  }
+}
+
+module "tls_secret" {
+  source          = "../../../modules/kubernetes/setup_tls_secret"
+  namespace       = kubernetes_namespace.echo.metadata[0].name
+  tls_secret_name = var.tls_secret_name
+}
+
+resource "kubernetes_deployment" "echo" {
+  metadata {
+    name      = "echo"
+    namespace = kubernetes_namespace.echo.metadata[0].name
+    labels = {
+      app  = "echo"
+      tier = var.tier
+    }
+  }
+  spec {
+    replicas = 5
+    selector {
+      match_labels = {
+        app = "echo"
+      }
+    }
+    template {
+      metadata {
+        labels = {
+          app = "echo"
+        }
+      }
+      spec {
+        container {
+          image = "mendhak/http-https-echo"
+          name  = "echo"
+          port {
+            container_port = 8080
+          }
+          port {
+            container_port = 8443
+          }
+        }
+      }
+    }
+  }
+}
+
+resource "kubernetes_service" "echo" {
+  metadata {
+    name      = "echo"
+    namespace = kubernetes_namespace.echo.metadata[0].name
+    labels = {
+      "app" = "echo"
+    }
+  }
+
+  spec {
+    selector = {
+      app = "echo"
+    }
+    port {
+      name        = "http"
+      port        = "80"
+      target_port = "8080"
+    }
+  }
+}
+
+module "ingress" {
+  source          = "../../../modules/kubernetes/ingress_factory"
+  namespace       = kubernetes_namespace.echo.metadata[0].name
+  name            = "echo"
+  tls_secret_name = var.tls_secret_name
+}