refactor: remove Langfuse observability integration

- Delete lib/langfuse.ts, instrumentation.ts
- Remove API routes: log-save, log-feedback
- Remove feedback buttons (thumbs up/down) from chat
- Remove sessionId tracking throughout codebase
- Remove @langfuse/*, @opentelemetry dependencies
- Clean up env.example

package.json

@@ -17,12 +17,8 @@
         "@ai-sdk/google": "^2.0.0",
         "@ai-sdk/openai": "^2.0.19",
         "@ai-sdk/react": "^2.0.22",
-        "@langfuse/client": "^4.4.9",
-        "@langfuse/otel": "^4.4.4",
-        "@langfuse/tracing": "^4.4.9",
         "@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-select": "^2.2.6",