refactor: remove Langfuse observability integration

- Delete lib/langfuse.ts, instrumentation.ts
- Remove API routes: log-save, log-feedback
- Remove feedback buttons (thumbs up/down) from chat
- Remove sessionId tracking throughout codebase
- Remove @langfuse/*, @opentelemetry dependencies
- Clean up env.example

lib/langfuse.ts

@@ -1,95 +0,0 @@
-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 });
-}