chore: add Apache 2.0 license and update gitignore (#68)

* 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.)

* 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

* fix: remove deprecated instrumentationHook (enabled by default in Next.js 15)

* chore: add Apache 2.0 license and update gitignore

package.json

@@ -1,6 +1,7 @@
 {
     "name": "next-ai-draw-io",
     "version": "0.2.0",
+    "license": "Apache-2.0",
     "private": true,
     "scripts": {
         "dev": "next dev --turbopack --port 6002",