Add threaded comment replies for admin and curator

- Add parentId field to LessonComment (self-referential FK, SetNull on delete)
- Show replies indented under parent comment with left border visual
- Add reply button (visible to admin/curator only) with inline textarea
- Only root comments shown in main list; replies nested below their parent
- Update comment count to include replies
- Server-side validation: only admin/curator can reply, parent must belong to same lesson

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -301,9 +301,12 @@ model LessonComment {
   text      String
   deleted   Boolean  @default(false)
   createdAt DateTime @default(now())
+  parentId  String?
 
-  lesson Lesson @relation(fields: [lessonId], references: [id], onDelete: Cascade)
-  user   User   @relation(fields: [userId], references: [id], onDelete: Cascade)
+  lesson  Lesson          @relation(fields: [lessonId], references: [id], onDelete: Cascade)
+  user    User            @relation(fields: [userId], references: [id], onDelete: Cascade)
+  parent  LessonComment?  @relation("CommentReplies", fields: [parentId], references: [id], onDelete: SetNull)
+  replies LessonComment[] @relation("CommentReplies")
 }
 
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