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/components/curator/audio-recorder.tsx

@@ -5,9 +5,10 @@ import { useState, useRef } from "react";
 interface AudioRecorderProps {
   value: string | null;
   onChange: (url: string | null) => void;
+  uploadUrl?: string;
 }
 
-export function AudioRecorder({ value, onChange }: AudioRecorderProps) {
+export function AudioRecorder({ value, onChange, uploadUrl = "/api/curator/audio-upload" }: AudioRecorderProps) {
   const [state, setState] = useState<"idle" | "recording" | "recorded" | "uploading">("idle");
   const [localUrl, setLocalUrl] = useState<string | null>(null);
   const mediaRecorder = useRef<MediaRecorder | null>(null);
@@ -46,7 +47,7 @@ export function AudioRecorder({ value, onChange }: AudioRecorderProps) {
     const blob = new Blob(chunks.current, { type: mimeType.current });
     const form = new FormData();
     form.append("file", blob, "audio-response.webm");
-    const res = await fetch("/api/curator/audio-upload", { method: "POST", body: form });
+    const res = await fetch(uploadUrl, { method: "POST", body: form });
     const data = await res.json();
     if (data.url) {
       onChange(data.url);