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/config.py

@@ -66,6 +66,12 @@ class Config:
     VAPID_PUBLIC_KEY: str = os.environ.get("VAPID_PUBLIC_KEY", "")
     VAPID_CLAIMS_SUB: str = os.environ.get("VAPID_CLAIMS_SUB", "mailto:admin@fabledassistant.local")
 
+    # Voice (Speech-to-Speech) feature
+    VOICE_ENABLED: bool = os.environ.get("VOICE_ENABLED", "").lower() in ("1", "true", "yes")
+    STT_BACKEND: str = os.environ.get("STT_BACKEND", "faster-whisper")
+    STT_MODEL: str = os.environ.get("STT_MODEL", "base.en")
+    TTS_BACKEND: str = os.environ.get("TTS_BACKEND", "kokoro")
+
     @classmethod
     def oidc_enabled(cls) -> bool:
         return bool(cls.OIDC_ISSUER and cls.OIDC_CLIENT_ID and cls.OIDC_CLIENT_SECRET)
@@ -93,5 +99,11 @@ class Config:
                 "SECRET_KEY is set to the insecure default but SECURE_COOKIES=true indicates "
                 "a production deployment. Set SECRET_KEY or SECRET_KEY_FILE before starting."
             )
+        _valid_stt_models = {"tiny.en", "base.en", "small.en", "medium.en"}
+        if cls.VOICE_ENABLED and cls.STT_MODEL not in _valid_stt_models:
+            errors.append(
+                f"STT_MODEL='{cls.STT_MODEL}' is not supported. "
+                f"Valid values: {', '.join(sorted(_valid_stt_models))}"
+            )
         if errors:
             raise ValueError("Configuration errors:\n" + "\n".join(f"  - {e}" for e in errors))