Add frontend health ping endpoint and project docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/app/health/ping/route.ts

@@ -0,0 +1,5 @@
+import { NextResponse } from "next/server";
+
+export async function GET() {
+  return NextResponse.json({ status: "pong" });
+}