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/config/env.ts

@@ -37,8 +37,11 @@ export const config = {
     contactTo: process.env.CONTACT_EMAIL_TO || '',
     contactFrom: process.env.CONTACT_EMAIL_FROM || '',
     smtpFrom: process.env.SMTP_FROM || '',
+    leaveNotify: process.env.LEAVE_NOTIFY_EMAIL || '',
   },
 
+  appUrl: process.env.APP_URL || '',
+
   cors: {
     origins: (process.env.CORS_ORIGINS || '').split(',').filter(Boolean),
   },