feat: add email notification for new leave requests

- mailer.ts: nodemailer transport via local sendmail
- leave-notification.ts: HTML email matching PHP template
- Sends notification to LEAVE_NOTIFY_EMAIL on new leave request
- Non-blocking: errors logged but don't fail the request
- Added LEAVE_NOTIFY_EMAIL and APP_URL env vars

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/mailer.ts

@@ -0,0 +1,25 @@
+import nodemailer from 'nodemailer';
+import { config } from '../config/env';
+
+const transporter = nodemailer.createTransport({
+  sendmail: true,
+  newline: 'unix',
+  path: '/usr/sbin/sendmail',
+});
+
+export async function sendMail(to: string, subject: string, html: string): Promise<boolean> {
+  const from = config.email.smtpFrom || config.email.contactFrom || 'web@boha-automation.cz';
+
+  try {
+    await transporter.sendMail({
+      from: `System <${from}>`,
+      to,
+      subject,
+      html,
+    });
+    return true;
+  } catch (err) {
+    console.error(`Mailer error: failed to send to ${to}:`, err);
+    return false;
+  }
+}