feat: complete German email template implementation and language detection

- Added comprehensive German translations for all email templates:
  - gallery_created: Welcome email with proper German formatting
  - expiration_warning: Warning email with countdown in German
  - gallery_expired: Expiration notice in German
  - archive_complete: Archive completion notice in German

- Enhanced language detection with priority system:
  1. Event-specific language setting (new 'language' column)
  2. Global email config default language
  3. Domain-based detection (.de, .at, .ch, .li)

- Added language preferences:
  - events.language column for per-event language settings
  - email_configs.default_language for system-wide default
  - Set default to German ('de') for existing installations

- Fixed email processor to:
  - Pass eventId for proper language detection
  - Handle both SQLite and PostgreSQL JSON formats
  - Log language used for each email sent

All German templates are professionally translated with proper:
- Greetings ("Liebe(r)" instead of "Dear")
- Date formatting (DD.MM.YYYY)
- Professional closings ("Mit freundlichen Grüßen")
- UI elements ("Galerie-Details:" instead of "Gallery Details:")

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

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

backend/src/services/emailProcessor.js

@@ -59,15 +59,37 @@ async function initializeTransporter(forceReinit = false) {
 }
 
 // Get the appropriate language for a recipient
-async function getRecipientLanguage(email) {
-  // For now, check if the email domain ends with .de
-  // In the future, this could check user preferences
-  if (email && email.endsWith('.de')) {
-    return 'de';
+async function getRecipientLanguage(email, eventId = null) {
+  // First priority: Check event language setting if eventId is provided
+  if (eventId) {
+    try {
+      const event = await db('events').where('id', eventId).first();
+      if (event && event.language) {
+        return event.language;
+      }
+    } catch (error) {
+      logger.error('Error fetching event language:', error);
+    }
   }
   
-  // Check if there's a saved preference for this email
-  // This could be expanded to check user preferences in the database
+  // Second priority: Check email configs for default language
+  try {
+    const emailConfig = await db('email_configs').first();
+    if (emailConfig && emailConfig.default_language) {
+      return emailConfig.default_language;
+    }
+  } catch (error) {
+    logger.error('Error fetching email config language:', error);
+  }
+  
+  // Third priority: Check if the email domain suggests German
+  if (email) {
+    const germanDomains = ['.de', '.at', '.ch', '.li'];
+    const domain = email.toLowerCase();
+    if (germanDomains.some(d => domain.endsWith(d))) {
+      return 'de';
+    }
+  }
   
   return 'en'; // Default to English
 }
@@ -300,8 +322,8 @@ async function sendTemplateEmail(to, templateKey, variables) {
       throw new Error('Email configuration not found');
     }
 
-    // Determine recipient language
-    const language = await getRecipientLanguage(to);
+    // Determine recipient language (pass eventId if available in variables)
+    const language = await getRecipientLanguage(to, variables.eventId || null);
     
     // Process template with variables
     const { subject, htmlBody, textBody } = await processTemplate(template, variables, language);
@@ -410,6 +432,8 @@ async function processEmailQueue() {
 // Queue an email for sending
 async function queueEmail(eventId, recipientEmail, emailType, emailData) {
   try {
+    // Add eventId to emailData for language detection
+    emailData.eventId = eventId;
     await db('email_queue').insert({
       event_id: eventId,
       recipient_email: recipientEmail,