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-save/route.ts

@@ -0,0 +1,65 @@
+import { getLangfuseClient } from '@/lib/langfuse';
+import { randomUUID } from 'crypto';
+import { z } from 'zod';
+
+const saveSchema = z.object({
+  filename: z.string().min(1).max(255),
+  format: z.enum(['drawio', 'png', 'svg']),
+  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 = saveSchema.parse(await req.json());
+  } catch {
+    return Response.json({ success: false, error: 'Invalid input' }, { status: 400 });
+  }
+
+  const { filename, format, sessionId } = data;
+
+  try {
+    const timestamp = new Date().toISOString();
+
+    // Find the most recent chat trace for this session to attach the save flag
+    const tracesResponse = await langfuse.api.trace.list({
+      sessionId,
+      limit: 1,
+    });
+
+    const traces = tracesResponse.data || [];
+    const latestTrace = traces[0];
+
+    if (latestTrace) {
+      // Add a score to the existing trace to flag that user saved
+      await langfuse.api.ingestion.batch({
+        batch: [
+          {
+            type: 'score-create',
+            id: randomUUID(),
+            timestamp,
+            body: {
+              id: randomUUID(),
+              traceId: latestTrace.id,
+              name: 'diagram-saved',
+              value: 1,
+              comment: `User saved diagram as ${filename}.${format}`,
+            },
+          },
+        ],
+      });
+    }
+    // If no trace found, skip logging (user hasn't chatted yet)
+
+    return Response.json({ success: true, logged: !!latestTrace });
+  } catch (error) {
+    console.error('Langfuse save error:', error);
+    return Response.json({ success: false, error: 'Failed to log save' }, { status: 500 });
+  }
+}