Enrich lesson comments: Markdown formatting, edit, higher limit

Add a formatting toolbar + safe Markdown rendering (react-markdown +
rehype-sanitize), raise the length limit 2000 -> 10000, and let authors
edit their own comments (new editComment action + editedAt column shown
as an 'изменено' badge).

prisma/schema.prisma

@@ -353,12 +353,13 @@ model HomeworkFeedback {
 // ─────────────────────────────────────────────
 
 model LessonComment {
-  id        String   @id @default(cuid())
+  id        String    @id @default(cuid())
   lessonId  String
   userId    String
   text      String
-  deleted   Boolean  @default(false)
-  createdAt DateTime @default(now())
+  deleted   Boolean   @default(false)
+  createdAt DateTime  @default(now())
+  editedAt  DateTime?
   parentId  String?
 
   lesson  Lesson          @relation(fields: [lessonId], references: [id], onDelete: Cascade)