Refactor code structure and remove redundant sections for improved readability and maintainability

lib/auth.js

@@ -0,0 +1,35 @@
+import { createHash } from 'node:crypto';
+
+function getRequiredEnv(name) {
+  const value = process.env[name];
+  if (!value) {
+    throw new Error(`Missing required environment variable: ${name}`);
+  }
+  return value;
+}
+
+export const ADMIN_USERNAME = getRequiredEnv('ADMIN_USERNAME');
+export const ADMIN_PASSWORD = getRequiredEnv('ADMIN_PASSWORD');
+export const ADMIN_SESSION_SECRET = getRequiredEnv('ADMIN_SESSION_SECRET');
+export const ADMIN_COOKIE = 'admin_session';
+
+function hash(value) {
+  return createHash('sha256').update(value).digest('hex');
+}
+
+function expectedToken() {
+  return hash(`${ADMIN_USERNAME}:${ADMIN_PASSWORD}:${ADMIN_SESSION_SECRET}`);
+}
+
+export function isValidAdminCredentials(username, password) {
+  return username === ADMIN_USERNAME && password === ADMIN_PASSWORD;
+}
+
+export function createAdminSessionToken() {
+  return expectedToken();
+}
+
+export function isAdminRequest(request) {
+  const token = request.cookies.get(ADMIN_COOKIE)?.value;
+  return token === expectedToken();
+}