[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/hackmd/main.tf

@@ -12,8 +12,8 @@ locals {
 }
 
 module "hackmd" {
-  source = "../../modules/kubernetes/hackmd"
-  hackmd_db_password             = var.hackmd_db_password
-  tls_secret_name                = var.tls_secret_name
-  tier                           = local.tiers.edge
+  source             = "./module"
+  hackmd_db_password = var.hackmd_db_password
+  tls_secret_name    = var.tls_secret_name
+  tier               = local.tiers.edge
 }