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

instrumentation.ts

@@ -1,35 +0,0 @@
-import { LangfuseSpanProcessor } from '@langfuse/otel';
-import { NodeTracerProvider } from '@opentelemetry/sdk-trace-node';
-
-export function register() {
-  // Skip telemetry if Langfuse env vars are not configured
-  if (!process.env.LANGFUSE_PUBLIC_KEY || !process.env.LANGFUSE_SECRET_KEY) {
-    console.warn('[Langfuse] Environment variables not configured - telemetry disabled');
-    return;
-  }
-
-  const langfuseSpanProcessor = new LangfuseSpanProcessor({
-    publicKey: process.env.LANGFUSE_PUBLIC_KEY,
-    secretKey: process.env.LANGFUSE_SECRET_KEY,
-    baseUrl: process.env.LANGFUSE_BASEURL,
-    // Filter out Next.js HTTP request spans so AI SDK spans become root traces
-    shouldExportSpan: ({ otelSpan }) => {
-      const spanName = otelSpan.name;
-      // Skip Next.js HTTP infrastructure spans
-      if (spanName.startsWith('POST /') ||
-          spanName.startsWith('GET /') ||
-          spanName.includes('BaseServer') ||
-          spanName.includes('handleRequest')) {
-        return false;
-      }
-      return true;
-    },
-  });
-
-  const tracerProvider = new NodeTracerProvider({
-    spanProcessors: [langfuseSpanProcessor],
-  });
-
-  // Register globally so AI SDK's telemetry also uses this processor
-  tracerProvider.register();
-}