Fix migration column and JSON errors

- Fixed migration 029: Use base email_templates columns (subject, body_html, body_text)
  instead of language-specific columns that don't exist yet
- Fixed migration 004: JSON.stringify the setting_value for app_settings table
- Removed German translations from backup email templates in core migration

The errors occurred because:
1. Migration 029 assumed language columns existed, but they're added by later migrations
2. Migration 004 passed a plain string to a JSON column in PostgreSQL

backend/migrations/legacy/004_add_categories_and_cms.js

@@ -40,7 +40,7 @@ async function up() {
   // Add language preference to app_settings for global default
   await db('app_settings').insert({
     setting_key: 'default_language',
-    setting_value: 'en',
+    setting_value: JSON.stringify('en'),
     setting_type: 'general',
     updated_at: new Date()
   });