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

services/academic-service/src/main/kotlin/ru/ysu/lms/academic/content/ContentService.kt

@@ -61,6 +61,16 @@ data class ContentImportResult(
     val assets: Int,
 )
 
+/**
+ * Новое расположение материалов: порядок глав, порядок разделов внутри глав
+ * (и перенос разделов между главами), порядок статей внутри разделов
+ * (и перенос статей между разделами). Применяется одним запросом.
+ */
+data class ReorderArticle(val id: String)
+data class ReorderSection(val id: String, val articles: List<ReorderArticle> = emptyList())
+data class ReorderChapter(val id: String, val sections: List<ReorderSection> = emptyList())
+data class ReorderTree(val chapters: List<ReorderChapter> = emptyList())
+
 private data class ParsedArticle(
     val path: String,
     val chapterPos: Int,
@@ -145,6 +155,36 @@ class ContentService(
         )
     }
 
+    /** Применяет новое расположение глав, разделов и статей (drag & drop в админке). */
+    @Transactional
+    fun reorder(tree: ReorderTree) {
+        tree.chapters.forEachIndexed { chapterIndex, chapterDto ->
+            val chapter = chapters.findById(uuidOrNull(chapterDto.id) ?: return@forEachIndexed).orElse(null)
+                ?: return@forEachIndexed
+            chapter.position = chapterIndex + 1
+            chapters.save(chapter)
+
+            chapterDto.sections.forEachIndexed { sectionIndex, sectionDto ->
+                val section = sections.findById(uuidOrNull(sectionDto.id) ?: return@forEachIndexed).orElse(null)
+                    ?: return@forEachIndexed
+                section.chapterId = chapter.id
+                section.position = sectionIndex + 1
+                sections.save(section)
+
+                sectionDto.articles.forEachIndexed { articleIndex, articleDto ->
+                    val article = articles.findById(uuidOrNull(articleDto.id) ?: return@forEachIndexed).orElse(null)
+                        ?: return@forEachIndexed
+                    article.sectionId = section.id
+                    article.position = articleIndex + 1
+                    articles.save(article)
+                }
+            }
+        }
+    }
+
+    private fun uuidOrNull(value: String): UUID? =
+        runCatching { UUID.fromString(value) }.getOrNull()
+
     @Transactional
     fun setChapterPublished(chapterId: UUID, published: Boolean) {
         val ch = chapters.findById(chapterId).orElseThrow { NotFoundException("Глава не найдена") }

services/academic-service/src/main/kotlin/ru/ysu/lms/academic/controller/ContentAdminController.kt

@@ -14,6 +14,7 @@ import org.springframework.web.bind.annotation.RestController
 import org.springframework.web.multipart.MultipartFile
 import ru.ysu.lms.academic.content.ContentImportResult
 import ru.ysu.lms.academic.content.ContentService
+import ru.ysu.lms.academic.content.ReorderTree
 import ru.ysu.lms.common.BadRequestException
 import java.util.UUID
 
@@ -71,6 +72,12 @@ class ContentAdminController(private val contentService: ContentService) {
         contentService.setArticlePublished(id, body.published)
     }
 
+    /** Новое расположение материалов (drag & drop глав, разделов и статей). */
+    @PostMapping("/reorder")
+    fun reorder(@RequestBody body: ReorderTree) {
+        contentService.reorder(body)
+    }
+
     @PostMapping("/import")
     fun import(@RequestParam("file") file: MultipartFile): ContentImportResult {
         if (file.isEmpty) throw BadRequestException("Выберите файл")