feat: integrate Langfuse for LLM observability (#66)

* 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)

package.json

@@ -16,8 +16,10 @@
         "@ai-sdk/google": "^2.0.0",
         "@ai-sdk/openai": "^2.0.19",
         "@ai-sdk/react": "^2.0.22",
+        "@langfuse/otel": "^4.4.4",
         "@next/third-parties": "^16.0.6",
         "@openrouter/ai-sdk-provider": "^1.2.3",
+        "@opentelemetry/sdk-trace-node": "^2.2.0",
         "@radix-ui/react-dialog": "^1.1.6",
         "@radix-ui/react-scroll-area": "^1.2.3",
         "@radix-ui/react-slot": "^1.1.2",