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

lib/langfuse.ts

@@ -0,0 +1,95 @@
+import { observe, updateActiveTrace } from '@langfuse/tracing';
+import { LangfuseClient } from '@langfuse/client';
+import * as api from '@opentelemetry/api';
+
+// Singleton LangfuseClient instance for direct API calls
+let langfuseClient: LangfuseClient | null = null;
+
+export function getLangfuseClient(): LangfuseClient | null {
+  if (!process.env.LANGFUSE_PUBLIC_KEY || !process.env.LANGFUSE_SECRET_KEY) {
+    return null;
+  }
+
+  if (!langfuseClient) {
+    langfuseClient = new LangfuseClient({
+      publicKey: process.env.LANGFUSE_PUBLIC_KEY,
+      secretKey: process.env.LANGFUSE_SECRET_KEY,
+      baseUrl: process.env.LANGFUSE_BASEURL,
+    });
+  }
+
+  return langfuseClient;
+}
+
+// Check if Langfuse is configured
+export function isLangfuseEnabled(): boolean {
+  return !!process.env.LANGFUSE_PUBLIC_KEY;
+}
+
+// Update trace with input data at the start of request
+export function setTraceInput(params: {
+  input: string;
+  sessionId?: string;
+  userId?: string;
+}) {
+  if (!isLangfuseEnabled()) return;
+
+  updateActiveTrace({
+    name: 'chat',
+    input: params.input,
+    sessionId: params.sessionId,
+    userId: params.userId,
+  });
+}
+
+// Update trace with output and end the span
+export function setTraceOutput(output: string, usage?: { promptTokens?: number; completionTokens?: number }) {
+  if (!isLangfuseEnabled()) return;
+
+  updateActiveTrace({ output });
+
+  const activeSpan = api.trace.getActiveSpan();
+  if (activeSpan) {
+    // Manually set usage attributes since AI SDK Bedrock streaming doesn't provide them
+    if (usage?.promptTokens) {
+      activeSpan.setAttribute('ai.usage.promptTokens', usage.promptTokens);
+      activeSpan.setAttribute('gen_ai.usage.input_tokens', usage.promptTokens);
+    }
+    if (usage?.completionTokens) {
+      activeSpan.setAttribute('ai.usage.completionTokens', usage.completionTokens);
+      activeSpan.setAttribute('gen_ai.usage.output_tokens', usage.completionTokens);
+    }
+    activeSpan.end();
+  }
+}
+
+// Get telemetry config for streamText
+export function getTelemetryConfig(params: {
+  sessionId?: string;
+  userId?: string;
+}) {
+  if (!isLangfuseEnabled()) return undefined;
+
+  return {
+    isEnabled: true,
+    // Disable automatic input recording to avoid uploading large base64 images to Langfuse media
+    // User text input is recorded manually via setTraceInput
+    recordInputs: false,
+    recordOutputs: true,
+    metadata: {
+      sessionId: params.sessionId,
+      userId: params.userId,
+    },
+  };
+}
+
+// Wrap a handler with Langfuse observe
+export function wrapWithObserve<T>(
+  handler: (req: Request) => Promise<T>
+): (req: Request) => Promise<T> {
+  if (!isLangfuseEnabled()) {
+    return handler;
+  }
+
+  return observe(handler, { name: 'chat', endOnExit: false });
+}