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/migrations/20260429100000_add_comment_replies/migration.sql

@@ -0,0 +1,4 @@
+ALTER TABLE "LessonComment" ADD COLUMN "parentId" TEXT;
+
+ALTER TABLE "LessonComment" ADD CONSTRAINT "LessonComment_parentId_fkey"
+  FOREIGN KEY ("parentId") REFERENCES "LessonComment"("id") ON DELETE SET NULL ON UPDATE CASCADE;