vault: mount arrstack engine + config + roles (3/3) (#127)

## Why

Creates the arrstack secrets engine itself: the mount + config and the per-scope roles that mint arrproxy API keys. **PR 3 of 3 (resources)**, stacked on #126 (policy). Final step of the register -> policy -> resources split (was #124).

## Change

- Adds `config/arrstack_secret_backend/arrstack.yaml`: mounts the engine at `arrstack` and writes its config (`base_url`, timeout). The arrproxy admin token stays out of git and is read from KV by the module.
- Adds `config/arrstack_secret_backend_role/arrstack/{all,sonarr,radarr,prowlarr}.yaml`: roles scoped to each arr app (plus one covering all three). Default `ttl` is **60s** (short-lived, renewed on demand); `max_ttl` 86400 mirrors the litellm sibling convention. The engine additionally caps renewal at the arrproxy admin token's fixed mint expiry.
- Adds `modules/vault_cluster/modules/arrstack_secret_backend{,_role}` and wires them in: `config/config.hcl` maps, `modules/vault_cluster/main.tf`, `variables.tf`, the environment inputs, and the root provider block.
- Provider source is `artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/vault-secrets-arrstack` (repo `terraform-provider-vault-secrets-arrstack`), local name `arrstack`.

## Apply order

Apply **after PR #126 (policy) AND after `terraform-provider-vault-secrets-arrstack` v0.1.0 is published** to the artifactapi terraform registry. Until the provider is published, `tofu init` cannot resolve it, so **CI/plan on this PR is red by design** — that is expected, not a regression.

Note **plan-green != apply-green**: the KV-sourced `admin_token` is only fetched at apply time, so a green plan does not prove the seeded token is readable.

## Stack

1. register -> #125
2. policy -> #126
3. **resources (this PR)** -> `benvin/arrstack-resources` off `benvin/arrstack-policy`

Supersedes #124.

---------

Co-authored-by: unkin-agent <unkin-agent@git.unkin.net>
Co-authored-by: BenVincent <benvin@main.unkin.net>
Reviewed-on: #127
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

modules/vault_cluster/variables.tf

@@ -316,6 +316,30 @@ variable "litellm_secret_backend_role" {
   default = {}
 }
 
+variable "arrstack_secret_backend" {
+  description = "Map of arrstack secret engines to create (mount + config). The arrproxy admin token is read from KV"
+  type = map(object({
+    plugin                  = optional(string, "vault-plugin-secrets-arrstack")
+    description             = optional(string)
+    base_url                = string
+    ca_cert                 = optional(string)
+    request_timeout_seconds = optional(number, 30)
+  }))
+  default = {}
+}
+
+variable "arrstack_secret_backend_role" {
+  description = "Map of arrstack roles to create"
+  type = map(object({
+    name    = string
+    backend = string
+    apps    = list(string)
+    ttl     = optional(number)
+    max_ttl = optional(number)
+  }))
+  default = {}
+}
+
 variable "plugins" {
   description = "Map of plugins to import (register) in the catalog, keyed by catalog name"
   type = map(object({