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)

components/chat-panel.tsx

@@ -61,6 +61,9 @@ export default function ChatPanel({
     const [showHistory, setShowHistory] = useState(false);
     const [input, setInput] = useState("");
 
+    // Generate a unique session ID for Langfuse tracing
+    const [sessionId, setSessionId] = useState(() => `session-${Date.now()}-${Math.random().toString(36).slice(2, 9)}`);
+
     const { messages, sendMessage, addToolResult, status, error, setMessages } =
         useChat({
             transport: new DefaultChatTransport({
@@ -177,6 +180,7 @@ Please retry with an adjusted search pattern or use display_diagram if retries a
                     {
                         body: {
                             xml: chartXml,
+                            sessionId,
                         },
                     }
                 );
@@ -293,6 +297,7 @@ Please retry with an adjusted search pattern or use display_diagram if retries a
                     onClearChat={() => {
                         setMessages([]);
                         clearDiagram();
+                        setSessionId(`session-${Date.now()}-${Math.random().toString(36).slice(2, 9)}`);
                     }}
                     files={files}
                     onFileChange={handleFileChange}