feat: manage litellm secrets engine via terraform-provider-litellmvaultsecret (#83)

## Why
The `vault-plugin-secrets-litellm` engine (mints LiteLLM virtual keys) is registered in Vault, but nothing in this repo declared its mount, config, or roles. This wires in the companion `litellm` provider (`git.unkin.net/unkin/litellmvaultsecret`) so the mount is managed as code alongside the other secret backends.

## Changes
- Add `litellm_secret_backend` module that mounts the engine and writes its config (`base_url`, `request_timeout_seconds`); reads the sensitive `master_key` from KV at `kv/service/vault/<country>/<region>/secret_backend/<path>`, matching the consul/kubernetes backend convention.
- Add `litellm_secret_backend_role` module that manages roles (`models`, `max_budget`, `key_alias_prefix`, `ttl`/`max_ttl` in seconds, `metadata`).
- Register both modules in `vault_cluster` `main.tf` and add typed variables in `variables.tf`.
- Discover `litellm_secret_backend[_role]` YAML in `config.hcl` and pass the maps through the terragrunt inputs.
- Declare the `litellm` provider (pinned `0.1.0`) and a `provider "litellm"` block in the generated root `backend.tf`.
- Add example config for the `litellm` mount and a sample `team-a` role.

## Notes
- Requires the `master_key` KV secret to exist at `kv/service/vault/au/syd1/secret_backend/litellm` before apply (the module reads it, does not create it).
- Assumes provider `git.unkin.net/unkin/litellmvaultsecret` `0.1.0` is published to the artifactapi `terraform-unkin` registry.

Reviewed-on: #83
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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"