Add app-token JWT secrets engine

Implement a generic Vault/OpenBao secrets engine that issues short-lived
signed JWTs for self-made services, replacing per-app static bearer
Secrets. Per-app roles set the audience, TTLs, subject allowlist and
custom claims; creds/<role> mints an EdDSA (or RS256) token. Apps
validate offline against the unauthenticated JWKS + OIDC-metadata paths,
so there is no Vault round-trip per request. Signing keys are seal-wrapped
in the barrier and rotate with a configurable JWKS grace window.

Mirrors the other vault-plugin-secrets-* engines: cmd ServeMultiplex,
Makefile with patch|minor|major tags, .woodpecker CI (k8s resources + SA),
dual-flavour nfpm RPM to artifactapi rpm-internal. Tests (-race) cover
issuance+JWKS validation for both algorithms, rotation grace/trim, role
isolation, subject allowlist and the unauthenticated/seal-wrap wiring.

Claude-Session: https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT

scripts/build-rpm.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+#
+# Package the (already built) plugin binary into RPMs with nfpm. Builds one RPM
+# per target server: Vault (/opt/vault-plugins) and OpenBao (/opt/openbao-plugins).
+# 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}"
+BINARY="vault-plugin-secrets-apptoken"
+DIST="dist"
+
+if [ ! -f "${DIST}/${BINARY}" ]; then
+  echo "ERROR: ${DIST}/${BINARY} not found; run 'make build' first" >&2
+  exit 1
+fi
+
+export PACKAGE_VERSION="${VERSION}"
+export PACKAGE_RELEASE="1"
+export PACKAGE_ARCH="amd64"
+export PACKAGE_PLATFORM="linux"
+export PACKAGE_DESCRIPTION="Vault/OpenBao secrets engine issuing short-lived signed JWT app tokens for self-made services"
+export PACKAGE_MAINTAINER="Ben Vincent <ben@unkin.net>"
+export PACKAGE_HOMEPAGE="https://git.unkin.net/unkin/vault-plugin-secrets-apptoken"
+export PACKAGE_LICENSE="MIT"
+
+build_flavor() {
+  export PACKAGE_NAME="$1"
+  export PACKAGE_PLUGIN_DIR="$2"
+  export PACKAGE_PREINSTALL="${DIST}/preinstall-${PACKAGE_NAME}.sh"
+  envsubst '${PACKAGE_PLUGIN_DIR}' < packaging/scripts/preinstall.sh.tmpl > "${PACKAGE_PREINSTALL}"
+  envsubst < packaging/nfpm.yaml > "${DIST}/nfpm-${PACKAGE_NAME}.yaml"
+  nfpm pkg --config "${DIST}/nfpm-${PACKAGE_NAME}.yaml" --target "${DIST}" --packager rpm
+}
+
+build_flavor "vault-plugin-secrets-apptoken"   "/opt/vault-plugins"
+build_flavor "openbao-plugin-secrets-apptoken" "/opt/openbao-plugins"
+
+echo "Built:"
+ls -1 "${DIST}"/*.rpm