implemented the Ticker module with a native ProseMirror document renderer and integrated API support for structured content, navigation trees, and proxied files

lib/api/errors/ticker_content_unavailable_exception.dart

@@ -0,0 +1,19 @@
+import 'app_exception.dart';
+
+/// Raised when a CONTENT ticker page has no native ProseMirror payload yet
+/// (404 `CONTENT_UNAVAILABLE`). Carries the site-relative [webUrl] so the UI
+/// can offer to open the page in the browser instead. Not retryable — the
+/// content only appears after an admin re-saves the page.
+class TickerContentUnavailableException extends AppException {
+  final String? webUrl;
+
+  const TickerContentUnavailableException({
+    this.webUrl,
+    super.technicalDetails,
+  }) : super(
+         userMessage:
+             'Diese Seite ist in der App noch nicht verfügbar. '
+             'Du kannst sie im Browser öffnen.',
+         allowRetry: false,
+       );
+}