Add server-side TTS via Lemonade audio/speech endpoint

[kovtcharov]

Summary

Lemonade v9.4.1 exposes Kokoro TTS via an OpenAI-compatible POST /api/v1/audio/speech endpoint with streaming support. GAIA currently loads Kokoro locally in Python (src/gaia/audio/kokoro_tts.py). Server-side TTS enables streaming audio playback (lower latency) and is now available on both Windows and Linux.

Reference

• Lemonade TTS example
• Scope doc

API

async with client.audio.speech.with_streaming_response.create(
    model="kokoro-v1",
    voice="coral",
    input="Today is a wonderful day!",
    stream_format="audio",
) as response:
    await LocalAudioPlayer().play(response)

Parameters: input (text), model (kokoro-v1), voice, speed, response_format (mp3/wav/opus/pcm), stream_format

Changes Required

File	Change	Effort
src/gaia/audio/lemonade_tts.py	New file. Create LemonadeTTS class wrapping /audio/speech. Implement generate_speech(text, voice, speed) with streaming support via AsyncOpenAI.audio.speech.with_streaming_response.create()	Medium
src/gaia/audio/audio_client.py	Add tts_backend config: "lemonade" (default) or "local" (fallback to KokoroTTS)	Medium
src/gaia/talk/sdk.py	Prefer Lemonade TTS when available; handle streaming audio playback	Medium
src/gaia/cli.py	Add `--tts-backend lemonade	localflag togaia talk`
tests/unit/test_lemonade_tts.py	Unit tests with mocked client	Medium

Key Design Decisions

• Streaming for talk: Use stream_format="audio" with PCM for lowest latency
• File output: Use buffered response with WAV format for file saves
• Voice mapping: Lemonade Kokoro voices (coral, etc.) may differ from GAIA's current voice list (af_bella, am_adam). Need to map or expose both.
• Keep existing KokoroTTS: Same fallback strategy as ASR — auto-detect, graceful fallback.

Acceptance Criteria

• LemonadeTTS class wraps Lemonade /audio/speech endpoint
• Streaming audio playback works in gaia talk
• AudioClient auto-detects Lemonade TTS availability
• TalkSDK prefers Lemonade TTS, falls back to local Kokoro
• --tts-backend CLI flag works
• Existing KokoroTTS continues to work unchanged
• Unit tests pass