Add coverImage poster to player, fix TipTap v3 editor reset, quiz admin preview

- Add coverImage field to Lesson model (prisma)
- Pass coverImage as poster prop to KinescopePlayer
- Show quiz in read-only preview mode for admin on lesson page
- Fix TipTap v3 editor reset on save: pass [lesson.id] as deps to useEditor
  to prevent setOptions() from reinitializing content on every re-render
- Replace saveLesson Server Action call with fetch PATCH /api/admin/lessons/[id]
  to avoid Next.js 16 automatic RSC refresh after Server Actions
- Simplify revalidatePath: only revalidate module page, not lesson editor page

src/app/api/admin/lessons/[lessonId]/route.ts

@@ -0,0 +1,34 @@
+import { NextRequest, NextResponse } from "next/server";
+import { headers } from "next/headers";
+import { auth } from "@/lib/auth";
+import { prisma } from "@/lib/prisma";
+
+export async function PATCH(
+  req: NextRequest,
+  { params }: { params: Promise<{ lessonId: string }> }
+) {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session || session.user.role !== "admin") {
+    return NextResponse.json({ error: "Forbidden" }, { status: 403 });
+  }
+
+  const { lessonId } = await params;
+  const body = await req.json() as {
+    title: string;
+    kinescopeId: string;
+    content: object;
+    published: boolean;
+  };
+
+  await prisma.lesson.update({
+    where: { id: lessonId },
+    data: {
+      title: body.title,
+      kinescopeId: body.kinescopeId || null,
+      content: body.content,
+      published: body.published,
+    },
+  });
+
+  return NextResponse.json({ ok: true });
+}