feat: integrate Langfuse for LLM observability

- Add instrumentation.ts with Langfuse OpenTelemetry exporter
- Enable experimental telemetry on streamText calls
- Add instrumentationHook to Next.js config
- Install required dependencies (@vercel/otel, langfuse-vercel, etc.)

instrumentation.ts

@@ -0,0 +1,9 @@
+import { registerOTel } from '@vercel/otel';
+import { LangfuseExporter } from 'langfuse-vercel';
+
+export function register() {
+  registerOTel({
+    serviceName: 'next-ai-draw-io',
+    traceExporter: new LangfuseExporter(),
+  });
+}