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/lib/actions/lesson-actions.ts

@@ -21,16 +21,29 @@ export async function saveLesson(
     published: boolean;
   }
 ) {
-  await requireAdmin();
-  await prisma.lesson.update({
-    where: { id: lessonId },
-    data: {
-      title: data.title,
-      kinescopeId: data.kinescopeId || null,
-      content: JSON.parse(JSON.stringify(data.content)),
-      published: data.published,
-    },
-  });
+  console.log("[saveLesson] start", lessonId);
+  try {
+    await requireAdmin();
+    console.log("[saveLesson] auth ok");
+  } catch (e) {
+    console.error("[saveLesson] auth failed:", e);
+    throw e;
+  }
+  try {
+    await prisma.lesson.update({
+      where: { id: lessonId },
+      data: {
+        title: data.title,
+        kinescopeId: data.kinescopeId || null,
+        content: JSON.parse(JSON.stringify(data.content)),
+        published: data.published,
+      },
+    });
+    console.log("[saveLesson] db update ok");
+  } catch (e) {
+    console.error("[saveLesson] db update failed:", e);
+    throw e;
+  }
   revalidatePath(`/admin/courses/${courseId}/modules/${moduleId}`);
-  revalidatePath(`/admin/courses/${courseId}/modules/${moduleId}/lessons/${lessonId}`);
+  console.log("[saveLesson] done");
 }