Content: drag & drop reordering and moving of chapters, sections and articles

- academic: POST /api/v1/content/reorder applies a full tree arrangement in one
  request — chapter order, section order and section->chapter moves, article
  order and article->section moves (positions and parent ids updated)
- web: POST /admin/content/reorder accepts the tree from JS (CSRF via query
  param), calls the gateway with the admin token
- /learn admin view: draggable handles (⠿) for chapters, sections and articles;
  drop before/after to reorder, drop into a chapter/section to move; visual
  indicators; structure is collected from DOM and saved, then page reloads
- docs: class diagram method + new sequence diagram for drag & drop

docs/03-domain-model.md

@@ -212,6 +212,7 @@ classDiagram
         +moveArticle(UUID, UUID) ContentArticleView
         +deleteArticle(UUID)
         +importZip(String, ByteArray) ContentImportResult
+        +reorder(ReorderTree)
     }
     class LabImportParser {
         +parse(String, ByteArray) LabImportBundle