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

.gitignore

@@ -0,0 +1,5 @@
+/dist/
+/vault-plugin-secrets-apptoken
+*.out
+*.test
+.env