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>

web/css/components/app-buttons.css

@@ -1,3 +1,4 @@
+@layer components {
 .danger:is(button, [type="submit"], [type="button"], [role="button"]) {
   --app-background-color: var(--app-notice-error-border-color);
   --app-border-color: var(--app-notice-error-border-color);
@@ -42,3 +43,4 @@
     var(--app-button-hover-box-shadow, 0 0 0 rgba(0, 0, 0, 0)),
     0 0 0 var(--app-outline-width) var(--app-form-element-invalid-focus-color);
 }
+}