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/services/email.service.ts

@@ -11,19 +11,17 @@ export interface EmailConfig {
   tls_reject_unauthorized: boolean;
 }
 
+export interface EmailTemplateTranslation {
+  subject: string;
+  body_html: string;
+  body_text?: string;
+}
+
 export interface EmailTemplate {
   id: number;
   template_key: string;
-  subject: string; // For backward compatibility
-  body_html: string; // For backward compatibility
-  body_text?: string; // For backward compatibility
-  subject_en: string;
-  subject_de: string;
-  body_html_en: string;
-  body_html_de: string;
-  body_text_en?: string;
-  body_text_de?: string;
   variables: string[];
+  translations: Record<string, EmailTemplateTranslation>;
   updated_at: string;
 }
 
@@ -63,16 +61,16 @@ export const emailService = {
   },
 
   // Update email template
-  async updateTemplate(key: string, template: Partial<EmailTemplate>): Promise<void> {
-    await api.put(`/admin/email/templates/${key}`, template);
+  async updateTemplate(key: string, data: { translations: Record<string, EmailTemplateTranslation> }): Promise<void> {
+    await api.put(`/admin/email/templates/${key}`, data);
   },
 
   // Preview email template
-  async previewTemplate(key: string, previewData: Record<string, string>, language: 'en' | 'de' = 'en'): Promise<EmailPreview> {
+  async previewTemplate(key: string, previewData: Record<string, string>, language: string = 'en'): Promise<EmailPreview> {
     const response = await api.post<EmailPreview>(
       `/admin/email/templates/${key}/preview`,
       { preview_data: previewData, language }
     );
     return response.data;
   }
-};
+};