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>

src/app/(student)/courses/[slug]/lessons/[lessonId]/page.tsx

@@ -56,9 +56,15 @@ export default async function LessonPage({ params }: Props) {
         })
       : null,
     prisma.lessonComment.findMany({
-      where: { lessonId },
+      where: { lessonId, parentId: null },
       orderBy: { createdAt: "asc" },
-      include: { user: { select: { id: true, name: true } } },
+      include: {
+        user: { select: { id: true, name: true } },
+        replies: {
+          orderBy: { createdAt: "asc" },
+          include: { user: { select: { id: true, name: true } } },
+        },
+      },
     }),
   ]);
 
@@ -232,7 +238,10 @@ export default async function LessonPage({ params }: Props) {
           style={{ borderTop: "2px solid var(--border)" }}
         >
           <p className="text-xs font-bold uppercase tracking-widest mb-6" style={{ color: "var(--muted-foreground)" }}>
-            Обсуждение ({comments.filter((c) => !c.deleted).length})
+            Обсуждение ({
+              comments.filter(c => !c.deleted).length +
+              comments.flatMap(c => c.replies).filter(r => !r.deleted).length
+            })
           </p>
           <LessonComments
             lessonId={lessonId}