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>

svxdr/go.mod

@@ -0,0 +1,16 @@
+module svxportal/svxdr
+
+go 1.24.4
+
+require (
+	github.com/eclipse/paho.mqtt.golang v1.5.1
+	github.com/redis/go-redis/v9 v9.17.2
+)
+
+require (
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
+	github.com/gorilla/websocket v1.5.3 // indirect
+	golang.org/x/net v0.44.0 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+)