feat: add SMTP mailer for price-change notifications

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/mailer.js

@@ -0,0 +1,30 @@
+const nodemailer = require('nodemailer');
+
+function createTransport(env = process.env) {
+  return nodemailer.createTransport({
+    host: env.SMTP_HOST,
+    port: Number(env.SMTP_PORT || 587),
+    secure: Number(env.SMTP_PORT) === 465,
+    auth: env.SMTP_USER ? { user: env.SMTP_USER, pass: env.SMTP_PASS } : undefined,
+  });
+}
+
+function formatPrice(value) {
+  return value.toFixed(2).replace('.', ',');
+}
+
+async function sendPriceChangeEmail(transport, { from, to, name, url, oldPrice, newPrice }) {
+  await transport.sendMail({
+    from,
+    to,
+    subject: `Prijs gewijzigd: ${name}`,
+    text: [
+      `De prijs van "${name}" is gewijzigd.`,
+      `Oude prijs: €${formatPrice(oldPrice)}`,
+      `Nieuwe prijs: €${formatPrice(newPrice)}`,
+      `Link: ${url}`,
+    ].join('\n'),
+  });
+}
+
+module.exports = { createTransport, sendPriceChangeEmail, formatPrice };