Stream browser-compatible media responses

[azurras]

What changed

• declare the media stream endpoint as ResponseEntity
• make Spring MVC use its async streaming handler for audio/mp4 responses
• verify an authenticated ready derivative starts and completes an async stream

Live evidence

• M4A conversion and cache are READY
• production logged HttpMessageNotWritableException because the wildcard response type selected an ordinary message converter

Verification

• 35/35 focused media service and controller tests pass