Hosted web classroom: microphone shows 'An internal error occurred' while local mic and Web Speech API work fine

Environment

• Hosted instance: https://openmaic.chat (web classroom)
• Browser: Microsoft Edge (latest, Chromium)
• OS: Windows desktop (no built-in mic, using Bluetooth headset Hands-Free profile)

Steps to reproduce

1. Open the hosted classroom page and log in
2. Click the microphone button to speak
3. A toast appears: "An internal error occurred. Please try again later."

What I verified locally (all working)

• Bluetooth headset is connected; the Hands-Free recording device is set as default in Windows (mmsys.cpl shows "Ready")
• getUserMedia audio capture works in the same browser (local test page, live volume meter)
• Web Speech API recognition works in the same browser (SpeechRecognition transcribed speech correctly, lang zh-CN)
• Microphone permission for the site is allowed in edge://settings/content/microphone
• Account quota is sufficient (about 5,300 credits remaining shown in UI)

Expected behavior

Voice input should be captured and transcribed in the classroom.

Actual behavior

Every attempt fails immediately with the generic "An internal error occurred" toast; no audio request seems to reach the backend.

Happy to provide more details (DevTools console/network logs) if needed.