Scaffold vault-plugin-secrets-arrstack engine

Mint dynamic arrproxy machine tokens via arrproxy's bearer-gated admin API
so Terraform-driven *arr onboarding can issue and revoke per-role tokens
non-interactively.

- Add backend, config, roles, creds paths and the arrstack_token secret
- Call POST/DELETE /api/admin/tokens with a vault:arrstack:<role> subject
- Enforce apps as a non-empty subset of sonarr/radarr/prowlarr
- Cap lease renewal at the arrproxy token's fixed expiry
- Add table-driven unit tests against a fake arrproxy admin server
- Add Makefile, nfpm packaging, and pre-commit/build/test/release pipelines

packaging/scripts/preinstall.sh.tmpl

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+# Ensure the plugin directory exists before the binary is laid down.
+# Rendered per flavour via envsubst (see scripts/build-rpm.sh).
+mkdir -p ${PACKAGE_PLUGIN_DIR}