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/pages/app-error.css

@@ -0,0 +1,38 @@
+.error-page {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  min-height: 100dvh;
+  padding: 2rem;
+  text-align: center;
+}
+
+.error-page__code {
+  font-size: 5rem;
+  font-weight: 700;
+  line-height: 1;
+  color: var(--color-text-muted);
+}
+
+.error-page__message {
+  margin-top: 1rem;
+  font-size: 1.125rem;
+  color: var(--color-text);
+}
+
+.error-page__back {
+  display: inline-block;
+  margin-top: 2rem;
+  padding: 0.5rem 1.25rem;
+  font-size: 0.875rem;
+  text-decoration: none;
+  border: 1px solid var(--color-border);
+  border-radius: 0.375rem;
+  color: var(--color-text);
+  background: var(--color-bg-surface);
+}
+
+.error-page__back:hover {
+  opacity: 0.8;
+}