feat(doorman): badge listener + decoder TDD + deploy scaffolding [#355]

Modern successor of the 2018 Perl doorman (snapshot in legacy/):
bash+coreutils HID decoder, HA webhook dispatch, systemd unit.
Offline decoder suite green (9 tests); shellcheck zero-warning.

Umbrella #345. Work item: https://projects.knownelement.com/issues/355

deploy/doorman.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=doorman - badge scan listener (KNEL/doorman)
+Documentation=https://git.knownelement.com/KNEL/doorman
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart=/opt/doorman/bin/doorman.sh
+EnvironmentFile=-/etc/default/doorman
+Restart=always
+RestartSec=5
+User=root
+# Runs as root only for raw /dev/input/* access on the reader Pi;
+# revisit with a dedicated input group once reader hardware is known.
+Nice=5
+
+[Install]
+WantedBy=multi-user.target