Consistent default loudness

Buffered synthesis now targets -16 dBFS peak by default. POST /v1/audio/speech, POST /v1/audio/speech/with-timestamps, and conversation requests normalise to that level without any extra options, so a clip from OpenAI and a clip from ElevenLabs come back at a consistent loudness instead of jumping between providers.

Pass your own volumeDbfs to target a different peak, for example -23 for more headroom. This is a peak-level target in dBFS, not an integrated-loudness (LUFS) measurement. The streaming endpoint POST /v1/audio/speech/stream is unchanged: it is a provider pass-through and does not accept volumeDbfs.