feat: add icecast config template + envsubst renderer

scripts/render-config.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+if [ ! -f config/icecast.env ]; then
+    echo "error: config/icecast.env missing — copy from icecast.env.example and fill in" >&2
+    exit 1
+fi
+
+set -a
+. ./config/icecast.env
+set +a
+
+envsubst '${ADMIN_PW} ${SOURCE_PW} ${RELAY_PW}' \
+    < config/icecast.xml.tmpl \
+    > config/icecast.xml
+
+chmod 0640 config/icecast.xml
+echo "rendered config/icecast.xml"