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

web/src/main/kotlin/ru/ysu/lms/web/admin/ContentAdminController.kt

@@ -38,9 +38,11 @@ class ContentAdminController(
         }
         return try {
             val result = api.importContent(token(session), file.originalFilename ?: "content.zip", file.bytes)
+            val removedNote = if (result.removed > 0) ", удалено устаревших: ${result.removed}" else ""
             redirect.addFlashAttribute(
                 "notice",
-                "Импортировано: глав ${result.chapters}, разделов ${result.sections}, статей ${result.articles}, картинок ${result.assets}",
+                "Импортировано: глав ${result.chapters}, разделов ${result.sections}, " +
+                    "статей ${result.articles}, картинок ${result.assets}$removedNote",
             )
             "redirect:/admin/content"
         } catch (ex: ApiException) {

web/src/main/kotlin/ru/ysu/lms/web/api/ApiDtos.kt

@@ -365,6 +365,7 @@ data class ContentImportResultDto(
     val sections: Int = 0,
     val articles: Int = 0,
     val assets: Int = 0,
+    val removed: Int = 0,
 )
 
 class DownloadDto(