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>

environments/root.hcl

@@ -11,6 +11,12 @@ provider "vault" {
   address = local.vault_addr
 }
 
+# The LiteLLM secrets engine is managed through its own provider, which talks to
+# the same Vault server. Token falls back to the VAULT_TOKEN environment variable.
+provider "litellm" {
+  address = local.vault_addr
+}
+
 terraform {
   backend "consul" {
     address = "https://consul.service.consul"
@@ -29,6 +35,10 @@ terraform {
       source  = "hashicorp/consul"
       version = "2.23.0"
     }
+    litellm = {
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/litellmvaultsecret"
+      version = "0.1.0"
+    }
   }
 }
 EOF