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/hooks/index.ts

@@ -1,4 +1,5 @@
 export * from './useSessionTimeout';
 export * from './useOnClickOutside';
 export * from './useLocalizedDate';
-export * from './useLocalizedTimeAgo';
+export * from './useLocalizedTimeAgo';
+export * from './usePermission';

frontend/src/hooks/usePermission.ts

@@ -0,0 +1,23 @@
+import { usePermissions } from '../contexts/PermissionsContext';
+
+/**
+ * Hook to check if the current user has a specific permission.
+ *
+ * @param permission - The permission to check
+ * @returns boolean indicating if the user has the permission
+ */
+export function usePermission(permission: string): boolean {
+  const { hasPermission } = usePermissions();
+  return hasPermission(permission);
+}
+
+/**
+ * Hook to check if the current user has any of the specified permissions.
+ *
+ * @param permissions - Array of permissions to check
+ * @returns boolean indicating if the user has any of the permissions
+ */
+export function useAnyPermission(permissions: string[]): boolean {
+  const { hasAnyPermission } = usePermissions();
+  return hasAnyPermission(permissions);
+}