- secret_backends.py: SecretBackend protocol, FileSecretBackend (historical),
OpenBaoBackend (KV-v2 read + file fallback), build_backend() env-selected
- _read_secret delegates to the active backend; default is file, so production
is unchanged until CASTLE_SECRET_BACKEND=openbao
- OpenBao token bootstraps from the file backend; missing/unreachable falls back
- tests: file hit/miss, backend selection, unreachable + empty-token fallback
Read path verified live against castle-openbao. Write path, auto-unseal-on-boot,
and TLS hardening documented as remaining for full OpenBao production use.
