feat: Add invitation email and improve email templates

- Add invitation email service with Slovak diacritics
- Move certificate email HTML template to separate file
- Add invitation email HTML/TXT templates
- Remove template caching for development flexibility
- Add send invitation endpoint (POST /registracie/:id/send-invitation)

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

src/routes/ai-kurzy.routes.js

@@ -180,4 +180,12 @@ router.post(
   aiKurzyController.sendCertificateEmail
 );
 
+// ==================== INVITATION EMAIL ====================
+
+router.post(
+  '/registracie/:registraciaId/send-invitation',
+  validateParams(registraciaIdSchema),
+  aiKurzyController.sendInvitationEmail
+);
+
 export default router;