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>

frontend/src/api/client.ts

@@ -619,3 +619,51 @@ export function getNewsItems(params: GetNewsItemsParams = {}) {
     `/api/briefing/news?${p}`
   )
 }
+
+// ─── Voice ────────────────────────────────────────────────────────────────────
+
+export interface VoiceStatusResult {
+  enabled: boolean
+  stt: boolean
+  tts: boolean
+  stt_model?: string
+  tts_backend?: string
+}
+
+export interface VoiceEntry {
+  id: string
+  label: string
+}
+
+export const getVoiceStatus = () => apiGet<VoiceStatusResult>('/api/voice/status')
+
+export const getVoiceList = () =>
+  apiGet<{ voices: VoiceEntry[] }>('/api/voice/voices').then(r => r.voices)
+
+export async function transcribeAudio(blob: Blob): Promise<{ transcript: string; duration_ms: number }> {
+  const form = new FormData()
+  form.append('audio', blob, 'audio.webm')
+  const res = await fetch('/api/voice/transcribe', { method: 'POST', body: form })
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({ error: `HTTP ${res.status}` }))
+    throw new ApiError(res.status, err)
+  }
+  return res.json()
+}
+
+export async function synthesiseSpeech(
+  text: string,
+  voice?: string,
+  speed?: number
+): Promise<Blob> {
+  const res = await fetch('/api/voice/synthesise', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ text, voice: voice ?? 'af_heart', speed: speed ?? 1.0 }),
+  })
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({ error: `HTTP ${res.status}` }))
+    throw new ApiError(res.status, err)
+  }
+  return res.blob()
+}