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/components/player/kinescope-player.tsx

@@ -4,15 +4,17 @@ import KinescopeReactPlayer from "@kinescope/react-kinescope-player";
 
 interface Props {
   videoId: string;
+  poster?: string;
 }
 
-export function KinescopePlayer({ videoId }: Props) {
+export function KinescopePlayer({ videoId, poster }: Props) {
   return (
     <div className="w-full aspect-video" style={{ border: "2px solid var(--border)" }}>
       <KinescopeReactPlayer
         videoId={videoId}
         width="100%"
         height="100%"
+        poster={poster}
       />
     </div>
   );