Implement multi-language email templates

- Add language columns to email_templates table (subject_en/de, body_html_en/de, body_text_en/de)
- Update adminEmail.js routes to support language-specific templates
- Create EmailProcessor service to handle language selection based on recipient
- Update EmailConfigPage component with language tabs similar to CMS pages
- Add German translations for all email templates
- Update all email queue usage to use proper template keys
- Add missing email templates (gallery_expired, archive_complete)
- Integrate email processor service into main server startup

The system now automatically selects the appropriate language (English/German) based on the recipient's email domain or preferences.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/services/email.service.ts

@@ -13,9 +13,15 @@ export interface EmailConfig {
 export interface EmailTemplate {
   id: number;
   template_key: string;
-  subject: string;
-  body_html: string;
-  body_text?: 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[];
   updated_at: string;
 }
@@ -61,10 +67,10 @@ export const emailService = {
   },
 
   // Preview email template
-  async previewTemplate(key: string, previewData: Record<string, string>): Promise<EmailPreview> {
+  async previewTemplate(key: string, previewData: Record<string, string>, language: 'en' | 'de' = 'en'): Promise<EmailPreview> {
     const response = await api.post<EmailPreview>(
       `/api/admin/email/templates/${key}/preview`,
-      { preview_data: previewData }
+      { preview_data: previewData, language }
     );
     return response.data;
   }