feat: add multi-administrator support with RBAC and fix backup/restore for S3

## Multi-Administrator System
- Add role-based access control (RBAC) with predefined roles (Super Admin, Admin, Editor, Viewer)
- Add granular permissions system for all admin operations
- Add admin user management page with invite functionality
- Add email invitation system for new administrators
- Add permission middleware protecting all admin routes
- Add PermissionGate component for frontend permission checks
- Track event creator (created_by) for audit purposes

## Backup & Restore Fixes
- Fix S3 backup: endpoint URL handling, manifest loading, field name compatibility
- Fix S3 restore: add list-backups endpoint, transform S3 config from frontend format
- Fix PostgreSQL compatibility: add .returning('id') for insert operations
- Fix disk space check: use df command, handle unknown space gracefully
- Fix dry-run validation to not block on warnings
- Fix req.user → req.admin in restore routes

## Database Migrations
- 054: Add roles table with predefined roles
- 055: Add permissions table
- 056: Add role_permissions junction table
- 057: Add role_id to admin_users
- 058: Add admin_invitations table
- 059: Add admin email templates
- 060: Add created_by to events table

## Other Improvements
- Update .gitignore to exclude planning docs and local backup directory
- Remove SQLite database file from tracking
- Add i18n translations for user management (EN/DE)

backend/server.js

@@ -436,6 +436,8 @@ app.use('/api/admin/photos', require('./src/routes/adminPhotos'));
 app.use('/api/admin/photo-export', require('./src/routes/adminPhotoExport'));
 app.use('/api/admin/css-templates', require('./src/routes/adminCssTemplates'));
 app.use('/api/admin/events', require('./src/routes/adminEventRename'));
+app.use('/api/admin/users', require('./src/routes/adminUsers'));
+app.use('/api/invite', require('./src/routes/acceptInvite'));
 app.use('/api/public/settings', require('./src/routes/publicSettings'));
 app.use('/api/public', require('./src/routes/publicCMS'));
 app.use('/api/images', require('./src/routes/protectedImages'));