fix: an update reboots the computer only once nobody is using it

The update check has been daily at 03:00, Persistent, since 0.1.5 — the
lab's installed stable machine logged it at 03:01 and 03:04 on successive
days — and it stays so. What changes is the reboot after an update. It
happened the moment the update was installed: harmless at 03:00, and a
room restarting itself mid-morning when a computer switched off at 03:00
caught up after being switched on.

Now the updater installs, with every check it had (the device's own
channel, a newer version, the manifest's SHA-256, name and version), and
starts classroom-reboot-when-idle, which reboots once no input device has
produced an event for 30 minutes and no sound card has played for as
long. At 03:00 that is about 03:30; switched off first, the next start is
on the new version. The service has no [Install] and nothing enables it.

The release gate checks after every install and upgrade that the timer is
enabled, daily at 03:00, Persistent, with no boot or repeat trigger and
no drop-in, and that the updater no longer reboots by itself.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

systemd/updater/ekklesia-classroom-reboot-when-idle.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Reboot after an Ekklesia Classroom update, once nobody is using the computer
+Documentation=file:///usr/local/bin/classroom-reboot-when-idle
+# Started by ekklesia-classroom-update after it installs an update that
+# needs a reboot; never at boot. A computer switched off before it
+# reboots simply starts on the new version.
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/classroom-reboot-when-idle