Add optional audio response for students in homework submissions

- Course: add allowAudio toggle (per-course setting, off by default)
- HomeworkSubmission: add audioUrl field
- Student: AudioRecorder in homework form when allowAudio is enabled
- Student: show audio player in submission view and curator feedback view
- Curator: show student audio on submission detail page
- AudioRecorder: accept uploadUrl prop (reused for student/curator)
- API: /api/student/audio-upload route for S3 upload

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/admin/courses/actions.ts

@@ -43,12 +43,13 @@ export async function updateCourse(courseId: string, formData: FormData) {
   const slug = formData.get("slug") as string;
   const description = (formData.get("description") as string) || null;
   const published = formData.get("published") === "true";
+  const allowAudio = formData.get("allowAudio") === "true";
   const coverImage = (formData.get("coverImage") as string) || null;
   const categoryId = (formData.get("categoryId") as string) || null;
 
   await prisma.course.update({
     where: { id: courseId },
-    data: { title, slug, description, published, coverImage, categoryId },
+    data: { title, slug, description, published, allowAudio, coverImage, categoryId },
   });
 
   revalidatePath("/admin/courses");