Allow image uploads in lesson comments

Add an image button to the comment toolbar backed by a new upload route
(images only, 5 MB, stored under comments/<userId>/), inserted as Markdown.

Only images served from our own storage are rendered: a third-party src would
be fetched by every reader of the lesson, exposing their address. Such links
are shown as plain links instead — this also defuses the external images a few
existing comments already carry.

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

@@ -289,6 +289,7 @@ export default async function LessonPage({ params }: Props) {
             comments={comments}
             currentUserId={session.user.id}
             currentUserRole={session.user.role ?? "student"}
+            imageHost={process.env.S3_CDN_URL ?? ""}
           />
         </div>
       )}