Phase 1 of Item #9 (Tower-stamped Secrets → ESO + OpenBao migration).
Replaces Tower's imperative kubectl-stamp of gitea-archive-pull with
a declarative ExternalSecret synced from OpenBao at v3/platform/gitea-
archive-pull. Other 4 Tower-stamped Secrets (cloudflare, s3-backup,
longhorn-s3, docker-mirror-pull) remain on legacy path.
Tower must pass externalSecrets.openbao.mountPath as a per-cluster
helm parameter (kubernetes-<server-name>) for ESO to activate; chart
guards against unset mountPath via {{ if }} in both new templates.
