Mint a short-lived Azure Speech token (STT/TTS)

Returns a 10-minute authorization token for direct Azure STT/TTS calls.

Why this pattern:

• The raw AZURE_SPEECH_KEY never leaves the server.
• Tokens expire in 10 minutes — limited blast radius if leaked.
• The frontend connects directly to Azure via WebSocket using the token, so audio streaming is realtime (no round-trip through this backend).

Frontend usage with Microsoft Speech SDK: const config = sdk.SpeechConfig.fromAuthorizationToken(token, region);