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

prisma/schema.prisma

@@ -130,6 +130,7 @@ model Lesson {
   title       String
   content     Json?
   kinescopeId String?
+  coverImage  String?
   order       Int      @default(0)
   published   Boolean  @default(false)
   createdAt   DateTime @default(now())