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/nginx-svxportal.conf

@@ -85,8 +85,8 @@ server {
   location /   { try_files $uri $uri/ =404; }
 
   # Live data API (/svxportal/api/status, /stream, /lastheard, /monitor-nodes,
-  # /local-trunks). The web UI expects a JSON/SSE backend here on 127.0.0.1:8098.
-  # That backend is NOT part of this repository — see INSTALL.md, section 8.
+  # /local-trunks), served by svxdr's nodelist module on 127.0.0.1:8098.
+  # See INSTALL.md, section 9.
   location ^~ /svxportal/api/ {
     proxy_pass http://127.0.0.1:8098/api/;
     proxy_http_version 1.1;