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/section.html

@@ -10,14 +10,20 @@
         <span class="nav-sep">/</span>
         <span th:text="${section.title}">Раздел</span>
     </nav>
-    <h1 class="page-title" th:text="${section.title}">Раздел</h1>
+    <h1 class="page-title">
+        <span th:text="${section.title}">Раздел</span>
+        <span th:if="${!section.published}" class="badge badge-warn ml-2">скрыто</span>
+    </h1>
     <div class="card card-flush">
         <table>
             <thead><tr><th>Статья</th></tr></thead>
             <tbody>
             <tr th:each="article : ${section.articles}">
-                <td><a th:href="@{/learn/{c}/{s}/{a}(c=${chapter.slug}, s=${section.slug}, a=${article.slug})}"
-                       th:text="${article.title}">—</a></td>
+                <td>
+                    <a th:href="@{/learn/{c}/{s}/{a}(c=${chapter.slug}, s=${section.slug}, a=${article.slug})}"
+                       th:text="${article.title}">—</a>
+                    <span th:if="${!article.published}" class="badge badge-warn ml-1">скрыто</span>
+                </td>
             </tr>
             <tr th:if="${#lists.isEmpty(section.articles)}"><td class="empty">Статей пока нет.</td></tr>
             </tbody>