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>

prisma/schema.prisma

@@ -96,6 +96,7 @@ model Course {
   description String?
   coverImage  String?
   published   Boolean  @default(false)
+  allowAudio  Boolean  @default(false)
   order       Int      @default(0)
   categoryId  String?
   createdAt   DateTime @default(now())
@@ -265,6 +266,7 @@ model HomeworkSubmission {
   files       Json?
   status      String    @default("PENDING") // PENDING | REVIEWING | APPROVED | REJECTED
   statusAt    DateTime?
+  audioUrl    String?
   submittedAt DateTime  @default(now())
 
   homework  Homework           @relation(fields: [homeworkId], references: [id], onDelete: Cascade)