Email: set Reply-To header from SMTP config

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

src/email.js

@@ -40,6 +40,7 @@ export async function sendPinEmail(to, name, pin) {
 
   await transport.sendMail({
     from,
+    replyTo: config.reply_to || undefined,
     to,
     subject: 'Your Hotel Number Four verification code',
     text: `Hi ${name},\n\nYour staff portal verification code is: ${pin}\n\nThis code expires in 15 minutes.\n\nIf you did not request this, please ignore this email.`,