add composer-unused, comprehensive docs, and project restructure

- Add icanhazstring/composer-unused as dev dependency for dependency hygiene checks
- Add German documentation (docs/) covering architecture, conventions, workflows, and developer checklists
- Add API layer (ApiAuth, ApiBootstrap, ApiResponse), audit, scheduler, custom fields, and SSO services
- Add Microsoft OIDC SSO, API token management, and user lifecycle features
- Add swagger-ui vendor integration and OpenAPI spec
- Add production Docker setup and bin/ scripts
- Update composer dependencies, config, templates, and frontend assets throughout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/Support/Guard.php

@@ -86,7 +86,7 @@ class Guard
                 echo json_encode(['error' => 'forbidden']);
                 exit;
             }
-            Router::redirect('error/forbidden');
+            Router::redirect('error/forbidden?url=' . urlencode(Request::pathWithQuery()));
         }
     }
 }