feat: add New Relic APM instrumentation to backend

Add Node.js New Relic agent with an on/off switch via NEW_RELIC_ENABLED
in .env. Uses a preload script (-r newrelic-preload.js) so the agent
loads before all other modules. Configured entirely through environment
variables — no newrelic.js config file needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/newrelic-preload.js

@@ -0,0 +1,7 @@
+// Conditionally load the New Relic agent before any other modules.
+// Controlled by the NEW_RELIC_ENABLED environment variable (.env).
+'use strict';
+
+if (process.env.NEW_RELIC_ENABLED === 'true') {
+  require('newrelic');
+}