feat: manage litellm secrets engine via terraform-provider-litellmvaultsecret

The vault-plugin-secrets-litellm engine mints LiteLLM virtual keys and is
now registered in Vault, but nothing declared its mount/config or roles in
this repo. Wire in the companion litellm provider so the mount is managed
as code alongside the other secret backends.

- Add litellm_secret_backend module: mounts the engine and writes its
  config (base_url, request_timeout_seconds); reads master_key from KV
- Add litellm_secret_backend_role module: manages roles (models,
  max_budget, key_alias_prefix, ttl/max_ttl seconds, metadata)
- Register both modules in vault_cluster main.tf and variables.tf
- Discover litellm_secret_backend[_role] YAML in config.hcl and pass them
  through terragrunt inputs
- Declare the litellm provider (git.unkin.net/unkin/litellmvaultsecret) and
  a provider block in the generated root backend.tf
- Add example config for the litellm mount and a sample role

modules/vault_cluster/main.tf

@@ -303,6 +303,37 @@ module "kubernetes_secret_backend_role" {
   depends_on = [module.kubernetes_secret_backend]
 }
 
+module "litellm_secret_backend" {
+  source = "./modules/litellm_secret_backend"
+
+  for_each = var.litellm_secret_backend
+
+  country                 = var.country
+  region                  = var.region
+  path                    = each.key
+  plugin                  = each.value.plugin
+  description             = each.value.description
+  base_url                = each.value.base_url
+  request_timeout_seconds = each.value.request_timeout_seconds
+}
+
+module "litellm_secret_backend_role" {
+  source = "./modules/litellm_secret_backend_role"
+
+  for_each = var.litellm_secret_backend_role
+
+  name             = each.value.name
+  backend          = each.value.backend
+  models           = each.value.models
+  max_budget       = each.value.max_budget
+  key_alias_prefix = each.value.key_alias_prefix
+  ttl              = each.value.ttl
+  max_ttl          = each.value.max_ttl
+  metadata         = each.value.metadata
+
+  depends_on = [module.litellm_secret_backend]
+}
+
 module "vault_policy" {
   source = "./modules/vault_policy"