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/article.html

@@ -13,7 +13,10 @@
         <span class="nav-sep">/</span>
         <span th:text="${article.title}">Статья</span>
     </nav>
-    <h1 class="page-title" th:text="${article.title}">Статья</h1>
+    <h1 class="page-title">
+        <span th:text="${article.title}">Статья</span>
+        <span th:if="${!article.published}" class="badge badge-warn ml-2">скрыто от студентов</span>
+    </h1>
     <div class="card card-body md-body" th:utext="${article.bodyHtml}"></div>
 </div>
 </body>