Content import: sync sections and articles (archive is the source of truth)

- importZip now updates titles/positions of existing chapters and sections and
  removes sections/articles of archived chapters that are missing in the archive
  (so restructured material no longer leaves stale sections behind); removed
  count is reported in ContentImportResult and in the admin flash message
- docs: note about import semantics in the class diagram and sequences

docs/03-domain-model.md

@@ -229,6 +229,8 @@ classDiagram
     class ContentSectionRepository
     class ContentArticleRepository
 
+    note for ContentService "Импорт ZIP — источник истины для глав из архива: устаревшие разделы и статьи удаляются (removed)"
+
     LabController --> LabService
     ContentPublicController --> ContentService
     ContentAdminController --> ContentService