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)

frontend/src/components/admin/RestoreWizard.jsx

@@ -500,13 +500,18 @@ export const RestoreWizard = () => {
               <div className="space-y-2 text-sm">
                 <div className="flex justify-between">
                   <span className="text-gray-600">{t('backup.restore.confirmation.spaceCheck.required')}:</span>
-                  <span className="font-medium">{formatBytes(validationResult.spaceCheck.required)}</span>
+                  <span className="font-medium">
+                    {validationResult.spaceCheck.requiredFormatted || formatBytes(validationResult.spaceCheck.required || 0)}
+                  </span>
                 </div>
                 <div className="flex justify-between">
                   <span className="text-gray-600">{t('backup.restore.confirmation.spaceCheck.available')}:</span>
-                  <span className="font-medium">{formatBytes(validationResult.spaceCheck.available)}</span>
+                  <span className="font-medium">
+                    {validationResult.spaceCheck.availableFormatted ||
+                     (validationResult.spaceCheck.available != null ? formatBytes(validationResult.spaceCheck.available) : t('common.unknown', 'Unknown'))}
+                  </span>
                 </div>
-                {!validationResult.spaceCheck.sufficient && (
+                {validationResult.spaceCheck.sufficient === false && (
                   <p className="text-red-600 text-xs mt-2">
                     <AlertCircle className="inline h-3 w-3 mr-1" />
                     {t('backup.restore.confirmation.spaceCheck.insufficient')}