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/Dockerfile.dev

@@ -7,6 +7,11 @@ RUN npm install
 
 COPY . .
 
+# New Relic agent — configured entirely via environment variables
+ENV NEW_RELIC_NO_CONFIG_FILE=true
+ENV NEW_RELIC_DISTRIBUTED_TRACING_ENABLED=true
+ENV NEW_RELIC_LOG=stdout
+
 EXPOSE 3000
 
 CMD ["npm", "run", "start:dev"]