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/au/syd1/terragrunt.hcl

@@ -68,6 +68,8 @@ inputs = {
   kubernetes_secret_backend      = local.config.kubernetes_secret_backend
   kubernetes_secret_backend_role = local.config.kubernetes_secret_backend_role
   pki_mount_only                 = local.config.pki_mount_only
+  litellm_secret_backend         = local.config.litellm_secret_backend
+  litellm_secret_backend_role    = local.config.litellm_secret_backend_role
 
   # Pass policy maps to vault_cluster module
   policy_auth_map  = local.policies.policy_auth_map