Add the netbox secrets engine modules and wiring (#115)

## Why

- Managing NetBox from Vault needs three capabilities the repo does not yet have: mounting the netbox engine, minting scoped tokens through roles, and creating the NetBox service users those roles mint tokens for. Landing the modules and config scaffolding before any backend or role data lets each concrete identity be added as pure data later.

## How

- Add three modules under `modules/vault_cluster/modules`: `netbox_secret_backend` (mount + engine config, admin token read from KV), `netbox_secret_backend_role` (mint ephemeral scoped tokens for a filename-derived NetBox username), and `netbox_user_management` (mirror consul_acl_management: read the seeded admin token, drive one e-breuninger/netbox provider per backend, and synthesize the NetBox user + object permissions from the role map's inline permissions).
- Derive the `netbox_secret_backend` and `netbox_secret_backend_role` maps in `config.hcl`, deriving each role's name and netbox_username from its filename so the engine role and NetBox username match by construction.
- Wire the three module blocks and their variables through `vault_cluster` and the syd1 terragrunt inputs, reusing the sanitized backend-alias pattern the Consul providers use.
- Leave the backend and role maps empty: the modules stand ready and create nothing until backend and role config data are added.

## Dependency order

- Stacked on the plugin registration PR (branch `benvin/netbox-plugin`); merge that first, then this, then the backend + role PR (#117). Plans clean with empty netbox maps.

---------

Co-authored-by: BenVincent <benvin@main.unkin.net>
Reviewed-on: #115
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

environments/au/syd1/terragrunt.hcl

@@ -39,6 +39,12 @@ locals {
     for backend_name, _ in local.config.consul_secret_backend :
     backend_name => replace(backend_name, "/", "_")
   }
+
+  # Same sanitized alias mapping for the NetBox providers.
+  netbox_backend_aliases = {
+    for backend_name, _ in local.config.netbox_secret_backend :
+    backend_name => replace(backend_name, "/", "_")
+  }
 }
 
 terraform {
@@ -81,10 +87,16 @@ inputs = {
   gitea_secret_backend      = local.config.gitea_secret_backend
   gitea_secret_backend_role = local.config.gitea_secret_backend_role
 
+  netbox_secret_backend      = local.config.netbox_secret_backend
+  netbox_secret_backend_role = local.config.netbox_secret_backend_role
+
   # Pass policy maps to vault_cluster module
   policy_auth_map  = local.policies.policy_auth_map
   policy_rules_map = local.policies.policy_rules_map
 
   # Pass sanitized consul backend aliases for provider configuration
   consul_backend_aliases = local.consul_backend_aliases
+
+  # Pass sanitized netbox backend aliases for provider configuration
+  netbox_backend_aliases = local.netbox_backend_aliases
 }