[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/webhook_handler/main.tf

@@ -18,7 +18,7 @@ locals {
 }
 
 module "webhook_handler" {
-  source = "../../modules/kubernetes/webhook_handler"
+  source = "./module"
   tls_secret_name                = var.tls_secret_name
   webhook_secret                 = var.webhook_handler_secret
   fb_verify_token                = var.webhook_handler_fb_verify_token