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/curator/homework/[submissionId]/page.tsx

@@ -160,6 +160,19 @@ export default async function SubmissionPage({ params }: Props) {
         )}
       </div>
 
+      {/* Student audio */}
+      {submission.audioUrl && (
+        <div className="mb-4">
+          <p
+            className="text-xs font-bold uppercase tracking-widest mb-2"
+            style={{ color: "var(--muted-foreground)" }}
+          >
+            Аудио студента
+          </p>
+          <audio controls src={submission.audioUrl} style={{ width: "100%", height: 40 }} />
+        </div>
+      )}
+
       {/* Student files */}
       {files.length > 0 && (
         <div className="mb-6">