Add src/lib/auth.ts helper for extracting user from Auth.js session

Implement task 5.2: Create getAuthUser() helper that calls auth() from
src/auth.ts and returns the authenticated user object or null if not
authenticated. Mark task 5.2 complete in tasks.md.

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

src/lib/auth.ts

@@ -0,0 +1,6 @@
+import { auth } from "@/auth";
+
+export async function getAuthUser() {
+  const session = await auth();
+  return session?.user ?? null;
+}