swarm-bao-forwarder-oidc still reads the secret from bao under the
forwarder's own leaf, but now writes it to /run/swarm-bao-forwarder-oidc on
the host (tmpfs), which the swarm-bao container bind-mounts read-only for the
collector's LoadCredential. Before, it wrote it to
/var/lib/nixos-containers/swarm-bao/var/lib/swarm-bao-otel-oidc/<id>.secret on
persistent disk, where a superseded secret stayed after rotation and went
into backups of the container root. Host tmpfiles now removes that old file.
Rotation is unchanged: the ten-minute timer rewrites the file when the value
differs and refresh_consumer restarts the forwarder. After a reboot the
secret is fetched again; the file is not pre-created, so until the store
answers the forwarder stays down (LoadCredential refuses a missing source) and
resumes from its journal cursor afterwards. The container now starts after
systemd-tmpfiles-resetup, since the new bind source is made there.
Part of #4998 (slice 3: forwarder).
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