feat: restore Langfuse observability integration (#103)

- Add lib/langfuse.ts with client, trace input/output, telemetry config
- Add instrumentation.ts for OpenTelemetry setup with Langfuse span processor
- Add /api/log-save endpoint for logging diagram saves
- Add /api/log-feedback endpoint for thumbs up/down feedback
- Update chat route with sessionId tracking and telemetry
- Add feedback buttons (thumbs up/down) to chat messages
- Add sessionId tracking throughout the app
- Update env.example with Langfuse configuration
- Add @langfuse/client, @langfuse/otel, @langfuse/tracing, @opentelemetry/sdk-trace-node

app/api/chat/route.ts

@@ -1,6 +1,7 @@
 import { streamText, convertToModelMessages, createUIMessageStream, createUIMessageStreamResponse } from 'ai';
 import { getAIModel } from '@/lib/ai-providers';
 import { findCachedResponse } from '@/lib/cached-responses';
+import { setTraceInput, setTraceOutput, getTelemetryConfig, wrapWithObserve } from '@/lib/langfuse';
 import { getSystemPrompt } from '@/lib/system-prompts';
 import { z } from "zod";
 
@@ -61,7 +62,27 @@ function createCachedStreamResponse(xml: string): Response {
 
 // Inner handler function
 async function handleChatRequest(req: Request): Promise<Response> {
-  const { messages, xml } = await req.json();
+  const { messages, xml, sessionId } = await req.json();
+
+  // Get user IP for Langfuse tracking
+  const forwardedFor = req.headers.get('x-forwarded-for');
+  const userId = forwardedFor?.split(',')[0]?.trim() || 'anonymous';
+
+  // Validate sessionId for Langfuse (must be string, max 200 chars)
+  const validSessionId = sessionId && typeof sessionId === 'string' && sessionId.length <= 200
+    ? sessionId
+    : undefined;
+
+  // Extract user input text for Langfuse trace
+  const currentMessage = messages[messages.length - 1];
+  const userInputText = currentMessage?.parts?.find((p: any) => p.type === 'text')?.text || '';
+
+  // Update Langfuse trace with input, session, and user
+  setTraceInput({
+    input: userInputText,
+    sessionId: validSessionId,
+    userId: userId,
+  });
 
   // === FILE VALIDATION START ===
   const fileValidation = validateFileParts(messages);
@@ -191,9 +212,19 @@ ${lastMessageText}
     messages: allMessages,
     ...(providerOptions && { providerOptions }),
     ...(headers && { headers }),
-    onFinish: ({ usage, providerMetadata }) => {
-      console.log('[Cache] providerMetadata:', JSON.stringify(providerMetadata, null, 2));
+    // Langfuse telemetry config (returns undefined if not configured)
+    ...(getTelemetryConfig({ sessionId: validSessionId, userId }) && {
+      experimental_telemetry: getTelemetryConfig({ sessionId: validSessionId, userId }),
+    }),
+    onFinish: ({ text, usage, providerMetadata }) => {
+      console.log('[Cache] Full providerMetadata:', JSON.stringify(providerMetadata, null, 2));
       console.log('[Cache] Usage:', JSON.stringify(usage, null, 2));
+      // Pass usage to Langfuse (Bedrock streaming doesn't auto-report tokens to telemetry)
+      // AI SDK uses inputTokens/outputTokens, Langfuse expects promptTokens/completionTokens
+      setTraceOutput(text, {
+        promptTokens: usage?.inputTokens,
+        completionTokens: usage?.outputTokens,
+      });
     },
     tools: {
       // Client-side tool that will be executed on the client
@@ -260,7 +291,8 @@ IMPORTANT: Keep edits concise:
   return result.toUIMessageStreamResponse();
 }
 
-export async function POST(req: Request) {
+// Wrap handler with error handling
+async function safeHandler(req: Request): Promise<Response> {
   try {
     return await handleChatRequest(req);
   } catch (error) {
@@ -268,3 +300,10 @@ export async function POST(req: Request) {
     return Response.json({ error: 'Internal server error' }, { status: 500 });
   }
 }
+
+// Wrap with Langfuse observe (if configured)
+const observedHandler = wrapWithObserve(safeHandler);
+
+export async function POST(req: Request) {
+  return observedHandler(req);
+}