Content: publish/hide chapters, sections and articles

- migration V5 adds published flag (default true) to chapters/sections/articles
- ContentService: tree(includeHidden) filters, treeAll(), findArticle(includeHidden),
  set*Published; public tree/article expose only published items
- admin API: GET /tree/all, GET /all/{chapter}/{section}/{article}, PUT
  /{chapters|sections|articles}/{id}/published
- web: learn pages for admins load the full tree, show "скрыто" badges and
  publish/hide buttons for every chapter, section and article; hidden articles
  open in preview for admins, guests get "not found"; article view carries
  published flag

web/src/main/resources/templates/learn/chapter.html

@@ -8,7 +8,10 @@
         <span class="nav-sep">/</span>
         <span th:text="${chapter.title}">Глава</span>
     </nav>
-    <h1 class="page-title" th:text="${chapter.title}">Глава</h1>
+    <h1 class="page-title">
+        <span th:text="${chapter.title}">Глава</span>
+        <span th:if="${!chapter.published}" class="badge badge-warn ml-2">скрыто</span>
+    </h1>
 
     <div th:if="${#lists.isEmpty(chapter.sections)}" class="card card-body text-muted">
         В этой главе пока нет разделов.
@@ -17,11 +20,13 @@
     <div th:each="section : ${chapter.sections}" class="card card-body mb-4">
         <h2 class="group-title">
             <a th:href="@{/learn/{c}/{s}(c=${chapter.slug}, s=${section.slug})}" th:text="${section.title}">Раздел</a>
+            <span th:if="${!section.published}" class="badge badge-warn">скрыто</span>
         </h2>
         <ul class="list-pad text-sm">
             <li th:each="article : ${section.articles}">
                 <a th:href="@{/learn/{c}/{s}/{a}(c=${chapter.slug}, s=${section.slug}, a=${article.slug})}"
                    th:text="${article.title}">Статья</a>
+                <span th:if="${!article.published}" class="badge badge-warn ml-1">скрыто</span>
             </li>
             <li th:if="${#lists.isEmpty(section.articles)}" class="text-muted">Статей пока нет</li>
         </ul>