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/log-feedback/route.ts

@@ -0,0 +1,103 @@
+import { getLangfuseClient } from '@/lib/langfuse';
+import { randomUUID } from 'crypto';
+import { z } from 'zod';
+
+const feedbackSchema = z.object({
+  messageId: z.string().min(1).max(200),
+  feedback: z.enum(['good', 'bad']),
+  sessionId: z.string().min(1).max(200).optional(),
+});
+
+export async function POST(req: Request) {
+  const langfuse = getLangfuseClient();
+  if (!langfuse) {
+    return Response.json({ success: true, logged: false });
+  }
+
+  // Validate input
+  let data;
+  try {
+    data = feedbackSchema.parse(await req.json());
+  } catch {
+    return Response.json({ success: false, error: 'Invalid input' }, { status: 400 });
+  }
+
+  const { messageId, feedback, sessionId } = data;
+
+  // Get user IP for tracking
+  const forwardedFor = req.headers.get('x-forwarded-for');
+  const userId = forwardedFor?.split(',')[0]?.trim() || 'anonymous';
+
+  try {
+    // Find the most recent chat trace for this session to attach the score to
+    const tracesResponse = await langfuse.api.trace.list({
+      sessionId,
+      limit: 1,
+    });
+
+    const traces = tracesResponse.data || [];
+    const latestTrace = traces[0];
+
+    if (!latestTrace) {
+      // No trace found for this session - create a standalone feedback trace
+      const traceId = randomUUID();
+      const timestamp = new Date().toISOString();
+
+      await langfuse.api.ingestion.batch({
+        batch: [
+          {
+            type: 'trace-create',
+            id: randomUUID(),
+            timestamp,
+            body: {
+              id: traceId,
+              name: 'user-feedback',
+              sessionId,
+              userId,
+              input: { messageId, feedback },
+              metadata: { source: 'feedback-button', note: 'standalone - no chat trace found' },
+              timestamp,
+            },
+          },
+          {
+            type: 'score-create',
+            id: randomUUID(),
+            timestamp,
+            body: {
+              id: randomUUID(),
+              traceId,
+              name: 'user-feedback',
+              value: feedback === 'good' ? 1 : 0,
+              comment: `User gave ${feedback} feedback`,
+            },
+          },
+        ],
+      });
+    } else {
+      // Attach score to the existing chat trace
+      const timestamp = new Date().toISOString();
+
+      await langfuse.api.ingestion.batch({
+        batch: [
+          {
+            type: 'score-create',
+            id: randomUUID(),
+            timestamp,
+            body: {
+              id: randomUUID(),
+              traceId: latestTrace.id,
+              name: 'user-feedback',
+              value: feedback === 'good' ? 1 : 0,
+              comment: `User gave ${feedback} feedback`,
+            },
+          },
+        ],
+      });
+    }
+
+    return Response.json({ success: true, logged: true });
+  } catch (error) {
+    console.error('Langfuse feedback error:', error);
+    return Response.json({ success: false, error: 'Failed to log feedback' }, { status: 500 });
+  }
+}