Add svxdr: the live-data backend (nodelist + proxy; astmeta/xlink optional), config example, systemd unit, INSTALL section 9

svxdr sources from the internal repo with AGPL headers; install/svxdr.json.example is a plain-reflector
layout (reflector 8099 -> proxy 8090 -> nodelist 8098) with CHANGEME secrets and no site values.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

install/systemd/svxdr.service

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+# svxdr — live-data daemon for SvxPortal. Install:
+#   sudo cp install/systemd/svxdr.service /etc/systemd/system/ && sudo systemctl daemon-reload && sudo systemctl enable --now svxdr
+# Build and configure per INSTALL.md section 9.
+[Unit]
+Description=svxdr — merged svx daemon (astmeta + nodelist + proxy + xlink)
+After=network-online.target redis-server.service
+Wants=network-online.target
+
+[Service]
+Type=simple
+# One binary, one config; which subsystems run is decided by "modules" in the
+# JSON (or override with flags, e.g. ExecStart=... --nodelist --proxy).
+ExecStart=/usr/local/bin/svxdr --config /etc/svxlink/svxdr.json
+Restart=on-failure
+RestartSec=3
+# Reload config in place (SIGHUP) without dropping the process.
+ExecReload=/bin/kill -HUP $MAINPID
+# Match the file/socket permissions the old services used (redis socket, logs,
+# /var/www network.json). Adjust User/Group to whatever nodelist ran as.
+#User=root
+LimitNOFILE=65536
+
+[Install]
+WantedBy=multi-user.target