refactor: Extract certificate template to separate file with template selection

- Move HTML template to src/templates/certificates/AIcertifikat.html
- Add template rendering with {{variable}} and {{#if}} syntax
- Add CERTIFICATE_TEMPLATES registry for multiple templates
- Accept templateName parameter in generateCertificate
- Add hasCertificate check to prevent duplicate certificates
- Add hasCertificate field to getCombinedTableData query

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

src/services/ai-kurzy/registracie.service.js

@@ -146,6 +146,7 @@ export const getCombinedTableData = async () => {
       poznamka: registracie.poznamka,
       createdAt: registracie.createdAt,
       dokumentyCount: sql`(SELECT COUNT(*) FROM prilohy WHERE registracia_id = ${registracie.id})::int`,
+      hasCertificate: sql`(SELECT COUNT(*) > 0 FROM prilohy WHERE registracia_id = ${registracie.id} AND typ_prilohy = 'certifikat')::boolean`,
     })
     .from(registracie)
     .innerJoin(ucastnici, eq(registracie.ucastnikId, ucastnici.id))