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 <[email protected]>

backend/migrations/027_add_language_preferences.js

@@ -0,0 +1,41 @@
+exports.up = async function(knex) {
+  // Add language column to events table if it doesn't exist
+  const hasLanguageInEvents = await knex.schema.hasColumn('events', 'language');
+  if (!hasLanguageInEvents) {
+    await knex.schema.alterTable('events', function(table) {
+      table.string('language', 5).defaultTo('en');
+    });
+  }
+  
+  // Add default_language to email_configs if it doesn't exist
+  const hasDefaultLanguage = await knex.schema.hasColumn('email_configs', 'default_language');
+  if (!hasDefaultLanguage) {
+    await knex.schema.alterTable('email_configs', function(table) {
+      table.string('default_language', 5).defaultTo('en');
+    });
+  }
+  
+  // Set default language to German for the existing email config
+  await knex('email_configs')
+    .update({
+      default_language: 'de'
+    });
+};
+
+exports.down = async function(knex) {
+  // Remove language column from events table
+  const hasLanguageInEvents = await knex.schema.hasColumn('events', 'language');
+  if (hasLanguageInEvents) {
+    await knex.schema.alterTable('events', function(table) {
+      table.dropColumn('language');
+    });
+  }
+  
+  // Remove default_language from email_configs
+  const hasDefaultLanguage = await knex.schema.hasColumn('email_configs', 'default_language');
+  if (hasDefaultLanguage) {
+    await knex.schema.alterTable('email_configs', function(table) {
+      table.dropColumn('default_language');
+    });
+  }
+};