feat: add optional Langfuse observability integration

- Add session tracking with unique sessionId per conversation
- Add user tracking via IP address (x-forwarded-for header)
- Make telemetry conditional - only enabled if LANGFUSE_PUBLIC_KEY is set
- Add environment variable validation in instrumentation.ts
- Add sessionId validation (type check + 200 char limit)
- Update env.example with Langfuse configuration docs
- Remove unused langfuse-vercel and @vercel/otel packages

instrumentation.ts

@@ -1,9 +1,22 @@
-import { registerOTel } from '@vercel/otel';
-import { LangfuseExporter } from 'langfuse-vercel';
+import { LangfuseSpanProcessor } from '@langfuse/otel';
+import { NodeTracerProvider } from '@opentelemetry/sdk-trace-node';
 
 export function register() {
-  registerOTel({
-    serviceName: 'next-ai-draw-io',
-    traceExporter: new LangfuseExporter(),
+  // Skip telemetry if Langfuse env vars are not configured
+  if (!process.env.LANGFUSE_PUBLIC_KEY || !process.env.LANGFUSE_SECRET_KEY) {
+    console.warn('[Langfuse] Environment variables not configured - telemetry disabled');
+    return;
+  }
+
+  const langfuseSpanProcessor = new LangfuseSpanProcessor({
+    publicKey: process.env.LANGFUSE_PUBLIC_KEY,
+    secretKey: process.env.LANGFUSE_SECRET_KEY,
+    baseUrl: process.env.LANGFUSE_BASEURL,
   });
+
+  const tracerProvider = new NodeTracerProvider({
+    spanProcessors: [langfuseSpanProcessor],
+  });
+
+  tracerProvider.register();
 }