Add on-tag RPM build (nfpm) and upload to artifactapi

Publish the plugin as an installable RPM so hosts can drop it into the Vault/
OpenBao plugin directory. On a tag, build the binary, package it with nfpm
(mirroring the rpmbuilder approach), and upload the RPM to artifactapi's local
rpm-internal repository.

- Add packaging/nfpm.yaml installing the binary to /opt/vault-plugins/ plus a
  preinstall script that creates the directory
- Add scripts/build-rpm.sh and make rpm / rpm-package targets
- Add .woodpecker/release.yml (event: tag): build -> nfpm package -> PUT to
  artifactapi remotes/rpm-internal/files/

scripts/build-rpm.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+#
+# Package the (already built) plugin binary into an RPM with nfpm.
+# Usage: scripts/build-rpm.sh [version]   (version defaults to $CI_COMMIT_TAG)
+#
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "${ROOT_DIR}"
+
+VERSION="${1:-${CI_COMMIT_TAG:-0.0.0-dev}}"
+VERSION="${VERSION#v}" # strip a leading v
+BINARY="vault-plugin-secrets-litellm"
+DIST="dist"
+
+if [ ! -f "${DIST}/${BINARY}" ]; then
+  echo "ERROR: ${DIST}/${BINARY} not found; run 'make build' first" >&2
+  exit 1
+fi
+
+export PACKAGE_NAME="${BINARY}"
+export PACKAGE_VERSION="${VERSION}"
+export PACKAGE_RELEASE="1"
+export PACKAGE_ARCH="amd64"
+export PACKAGE_PLATFORM="linux"
+export PACKAGE_DESCRIPTION="Vault/OpenBao dynamic secrets engine for LiteLLM virtual keys"
+export PACKAGE_MAINTAINER="Ben Vincent <ben@unkin.net>"
+export PACKAGE_HOMEPAGE="https://git.unkin.net/unkin/vault-plugin-secrets-litellm"
+export PACKAGE_LICENSE="MIT"
+
+envsubst < packaging/nfpm.yaml > "${DIST}/nfpm.yaml"
+nfpm pkg --config "${DIST}/nfpm.yaml" --target "${DIST}" --packager rpm
+
+echo "Built:"
+ls -1 "${DIST}"/*.rpm