feat: voice S2S — faster-whisper STT, Kokoro TTS, PTT overlay

Implements full speech-to-speech pipeline (all 4 phases):

Backend (Phase 1):
- services/stt.py: lazy WhisperModel singleton, run_in_executor transcription
- services/tts.py: lazy KPipeline singleton, WAV synthesis at 24kHz/16-bit
- routes/voice.py: /api/voice/status, /voices, /transcribe, /synthesise
- config.py: VOICE_ENABLED, STT_BACKEND, STT_MODEL, TTS_BACKEND env vars
- app.py: load STT/TTS models at startup when VOICE_ENABLED=true
- llm.py: voice_mode + voice_speech_style params inject speak-naturally prefix
- generation_task.py: voice_mode passed through from chat route
- chat.py: "voice" conversation type allowed + excluded from retention cleanup
- pyproject.toml + Dockerfile: faster-whisper, kokoro, soundfile dependencies

Frontend (Phases 2–4):
- composables/useVoiceRecorder.ts: MediaRecorder PTT wrapper
- composables/useVoiceAudio.ts: AudioContext WAV playback wrapper
- BriefingView.vue: Listen button (TTS read-aloud), auto-TTS mode, mic PTT
- VoiceOverlay.vue: global floating PTT button; creates/reuses voice conv;
  full record→transcribe→stream→TTS flow; Space bar hold-to-talk via App.vue
- SettingsView.vue: Voice tab (status badge, speech style, voice/speed)
- App.vue: mounts VoiceOverlay; Space keydown/keyup fires voice:ptt-toggle
- api/client.ts: getVoiceStatus, getVoiceList, transcribeAudio, synthesiseSpeech

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/app.py

@@ -31,6 +31,7 @@ from fabledassistant.routes.users import users_bp
 from fabledassistant.routes.api_keys import api_keys_bp
 from fabledassistant.routes.events import events_bp
 from fabledassistant.routes.search import search_bp
+from fabledassistant.routes.voice import voice_bp
 
 STATIC_DIR = Path(__file__).parent / "static"
 logger = logging.getLogger(__name__)
@@ -92,6 +93,7 @@ def create_app() -> Quart:
     app.register_blueprint(api_keys_bp)
     app.register_blueprint(events_bp)
     app.register_blueprint(search_bp)
+    app.register_blueprint(voice_bp)
 
     @app.before_request
     async def before_request():
@@ -264,6 +266,13 @@ def create_app() -> Quart:
         from fabledassistant.services.briefing_scheduler import start_briefing_scheduler
         await start_briefing_scheduler(asyncio.get_running_loop())
 
+        # Voice model loading (only when feature is enabled)
+        if Config.VOICE_ENABLED:
+            from fabledassistant.services.stt import load_stt_model
+            from fabledassistant.services.tts import load_tts_model
+            asyncio.create_task(load_stt_model())
+            asyncio.create_task(load_tts_model())
+
     @app.after_serving
     async def shutdown():
         from fabledassistant.services.briefing_scheduler import stop_briefing_scheduler