Email queue + async sending; legal settings and placeholders

- Add in-memory email queue with rate limiting (MAX_EMAILS_PER_HOUR)
- Bulk send to event attendees now queues and returns immediately
- Frontend shows 'Emails are being sent in the background'
- Legal pages, settings, and placeholders updates

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/admin/legal-pages/page.tsx

@@ -421,6 +421,46 @@ export default function AdminLegalPagesPage() {
                 </li>
               </ul>
             </div>
+
+            {/* Available Placeholders */}
+            <div className="bg-blue-50 border border-blue-200 rounded-lg p-4 text-sm text-blue-800">
+              <p className="font-medium mb-2">
+                {locale === 'es' ? 'Marcadores de posición disponibles:' : 'Available placeholders:'}
+              </p>
+              <p className="text-blue-700 mb-3">
+                {locale === 'es'
+                  ? 'Puedes usar estos marcadores en el contenido. Se reemplazarán automáticamente con los valores configurados en'
+                  : 'You can use these placeholders in the content. They will be automatically replaced with the values configured in'
+                }
+                {' '}
+                <a href="/admin/settings" className="underline font-medium hover:text-blue-900">
+                  {locale === 'es' ? 'Configuración > Legal' : 'Settings > Legal Settings'}
+                </a>.
+              </p>
+              <div className="grid grid-cols-1 sm:grid-cols-2 gap-x-6 gap-y-1.5">
+                {[
+                  { placeholder: '{{COMPANY_NAME}}', label: locale === 'es' ? 'Nombre de la empresa' : 'Company name' },
+                  { placeholder: '{{LEGAL_ENTITY_NAME}}', label: locale === 'es' ? 'Nombre de la entidad legal' : 'Legal entity name' },
+                  { placeholder: '{{RUC_NUMBER}}', label: locale === 'es' ? 'Número de RUC' : 'RUC number' },
+                  { placeholder: '{{COMPANY_ADDRESS}}', label: locale === 'es' ? 'Dirección de la empresa' : 'Company address' },
+                  { placeholder: '{{COMPANY_CITY}}', label: locale === 'es' ? 'Ciudad' : 'City' },
+                  { placeholder: '{{COMPANY_COUNTRY}}', label: locale === 'es' ? 'País' : 'Country' },
+                  { placeholder: '{{SUPPORT_EMAIL}}', label: locale === 'es' ? 'Email de soporte' : 'Support email' },
+                  { placeholder: '{{LEGAL_EMAIL}}', label: locale === 'es' ? 'Email legal' : 'Legal email' },
+                  { placeholder: '{{GOVERNING_LAW}}', label: locale === 'es' ? 'Ley aplicable' : 'Governing law' },
+                  { placeholder: '{{JURISDICTION_CITY}}', label: locale === 'es' ? 'Ciudad de jurisdicción' : 'Jurisdiction city' },
+                  { placeholder: '{{CURRENT_YEAR}}', label: locale === 'es' ? 'Año actual (automático)' : 'Current year (automatic)' },
+                  { placeholder: '{{LAST_UPDATED_DATE}}', label: locale === 'es' ? 'Fecha de última actualización (automático)' : 'Last updated date (automatic)' },
+                ].map(({ placeholder, label }) => (
+                  <div key={placeholder} className="flex items-center gap-2">
+                    <code className="bg-blue-100 text-blue-900 px-1.5 py-0.5 rounded text-xs font-mono whitespace-nowrap">
+                      {placeholder}
+                    </code>
+                    <span className="text-blue-700 text-xs truncate">{label}</span>
+                  </div>
+                ))}
+              </div>
+            </div>
           </div>
         </Card>
       </div>