Per iris's recommendation on #644 [comment 8043](http://localhost:3000/hyperhive/hyperhive/issues/644#issuecomment-8043):
swap the `chown root:tuwunel + chmod 0640 + pinned GID 10042` shape
(shipped via #649) for systemd's `LoadCredential=` mechanism.
How it works: systemd reads the host-side file at service start,
copies it into a per-service credentials dir
(`/run/credentials/tuwunel.service/registration_token`) owned by
the dynamic user with mode 0400. Service reads from there. All the
namespace mapping happens transparently inside systemd — keeps
`DynamicUser=true` + `PrivateUsers=true` intact.
Net diff from current shape:
- DROP `users.groups.tuwunel.gid = 10042;` from BOTH host AND container
- DROP `chown root:tuwunel "$tokenFile"; chmod 0640 "$tokenFile"`
from activation script; replace with `chmod 0600` (root:root)
- DROP `[ "var" "users" ]` activation dep on `users` (no longer
needs the group to exist before chown)
- ADD `systemd.services.tuwunel.serviceConfig.LoadCredential = [...]`
inside the container config
- CHANGE `registration_token_file` from the bind-mount path to
`/run/credentials/tuwunel.service/registration_token`
- KEEP the bind mount + activation-script token generation (load
credential reads the bind-mounted host file at service start)
Verified via `nix eval`:
- host: no `users.groups.tuwunel` (was: gid = 10042)
- container: tuwunel group exists with `gid = null` (auto-allocated;
no longer pinned to match host since it doesn't need to)
- container: tuwunel.service.serviceConfig.LoadCredential =
`["registration_token:/var/lib/hyperhive/matrix-register-token"]`
- container: services.matrix-tuwunel.settings.global.registration_token_file =
`/run/credentials/tuwunel.service/registration_token`
`/run/credentials/<service>/<id>` is a systemd-stable path
(documented in `man systemd.exec` → LoadCredential); safe to
hardcode.
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