feat: Add certificate email sending feature

- Create certificate-email.service.js with HTML email template
- Add beautiful gradient email template with course details
- Support PDF attachment via JMAP
- Add POST /prilohy/:prilohaId/send-email endpoint
- Add sendCertificateEmail controller function

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/routes/ai-kurzy.routes.js

@@ -174,4 +174,10 @@ router.post(
   aiKurzyController.generateCertificate
 );
 
+router.post(
+  '/prilohy/:prilohaId/send-email',
+  validateParams(prilohaIdSchema),
+  aiKurzyController.sendCertificateEmail
+);
+
 export default router;