Releases on Gitea, one-line installer and auto-update (1.1.0)

- .github/workflows/release.yml: a tag vX.Y.Z runs the tests, publishes
  daynivo-kiosk-os-X.Y.Z.tar.gz (+ .sha256) and install-online.sh as a
  Gitea release, then builds the SD image with pi-gen and attaches it.
- install-online.sh: curl | sudo bash installer for the latest release.
- voxip-kiosk-updater: checks the latest release, verifies and unpacks a
  newer one under /opt/daynivo-kiosk-os and runs its update.sh. A daily
  timer checks around 03:00 and installs when auto_update is on.
- Web interface: Systeem -> Software-updates with the auto-update switch,
  check now and install now (helper verbs update-check / update-install).
- CI checks out with plain git, so it runs on Gitea's act_runner too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

systemd/voxip-kiosk-update.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Daynivo Kiosk OS: controleren op updates (en installeren als dat aan staat)
+Documentation=file:///usr/share/doc/voxip-kiosk/README.md
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/lib/voxip-kiosk/voxip-kiosk-updater --auto
+TimeoutStartSec=1h
+SyslogIdentifier=voxip-kiosk-updater

systemd/voxip-kiosk-update.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Daynivo Kiosk OS: dagelijks controleren op updates
+
+[Timer]
+# Night-time, spread over two hours so a fleet doesn't hit the release server at once.
+OnCalendar=*-*-* 03:00
+RandomizedDelaySec=2h
+Persistent=false
+
+[Install]
+WantedBy=timers.target