feat: multilingual email templates with translations table

Replace column-based email template languages (subject_en/subject_de) with
a normalized email_template_translations table where each language is a row.
This allows adding new languages without schema changes.

- Add migration 075 to create email_template_translations table, migrate
  existing EN/DE data, and seed NL/PT/RU for customer-facing templates
- Update processTemplate() to query translations table with fallback chain
  (requested lang -> en -> first available), with legacy column fallback
- Restructure admin email API to return/accept translations object format
- Update frontend EmailConfigPage with dynamic 5-language tabs, translation
  count badges, and copy-from-language feature for empty translations
- Add Dutch to default language dropdown in general settings
- Add Dutch to clientAccessI18n and password security messages in emails
- Expand email domain detection for NL/BE/BR/PT/RU domains
- Add email i18n keys (copyFrom, noTranslation, etc.) across all 5 locales

frontend/src/features/settings/tabs/GeneralTab.tsx

@@ -246,6 +246,7 @@ export const GeneralTab: React.FC<GeneralTabProps> = ({
             >
               <option value="en">English</option>
               <option value="de">Deutsch</option>
+              <option value="nl">Nederlands</option>
               <option value="pt">Português (Brasil)</option>
               <option value="ru">Русский</option>
             </select>