[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/backend.tf

@@ -0,0 +1,6 @@
+# Generated by Terragrunt. Sig: nIlQXj57tbuaRZEa
+terraform {
+  backend "local" {
+    path = "/Users/viktorbarzin/code/infra/state/stacks/webhook_handler/terraform.tfstate"
+  }
+}