Screen: switch off by itself when nothing is going on

With `general.screen.off_command` set, the monitor switches off once nothing has
played and nobody has done anything for `auto_off_minutes` (15 by default, 0 for
never, editable in parent mode). "Doing anything" is any intent a person could
have caused: a remote key, a web command, or a key press or tap in the kiosk,
which the page reports at most every 30s via POST /api/screen/activity. The first
such intent after an auto-off runs `on_command`, so the remote wakes the screen
too. The auto-off itself is silent; switches a person asks for still blubb.

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

python-backend/config.yml.example

@@ -101,8 +101,15 @@ general:
   # device without one and the key does nothing. The command runs on every press, split
   # shell-style but not through a shell; it decides itself whether that means on or off
   # (ansible's pi_kiosk installs kiosk-screen for this).
+  #
+  # With an off_command the screen also switches itself off once nothing has played and
+  # nobody has touched the remote, the web UI or the kiosk keyboard for auto_off_minutes
+  # (editable in parent mode; 0 = never). The next press after that runs on_command.
   # screen:
   #   toggle_command: "/usr/local/bin/kiosk-screen toggle"
+  #   off_command: "/usr/local/bin/kiosk-screen off"
+  #   on_command: "/usr/local/bin/kiosk-screen on"
+  #   auto_off_minutes: 15
   #   timeout: 5.0               # seconds before a hung command is killed
 
   # Home Assistant integration. Omit the whole section to run without MQTT.