feat(tts): add pocket-tts engine alongside kokoro-fastapi

- New backend/internal/pockettts package: POST /tts client for
  kyutai-labs/pocket-tts; streams WAV response and transcodes to MP3
  via ffmpeg so the rest of the pipeline stays format-agnostic
- config.PocketTTS struct + POCKET_TTS_URL env var
- runner.Dependencies.PocketTTS field; runAudioTask routes by voice name:
  pockettts.IsPocketTTSVoice (alba, marius, javert, …) → pocket-tts,
  everything else → kokoro-fastapi
- Dockerfile: runner stage switched from distroless/static to alpine:3.21
  with ffmpeg + ca-certificates installed
- homelab compose: runner gets POCKET_TTS_URL=http://pocket-tts:8000
- Doppler prd + prd_homelab: KOKORO_URL=https://tts.libnovel.cc,
  POCKET_TTS_URL=https://pocket-tts.libnovel.cc

homelab/docker-compose.yml

@@ -61,6 +61,8 @@ services:
       KOKORO_URL: "http://kokoro-fastapi:8880"
       KOKORO_VOICE: "${KOKORO_VOICE}"
 
+      POCKET_TTS_URL: "http://pocket-tts:8000"
+
       RUNNER_WORKER_ID: "${RUNNER_WORKER_ID}"
       RUNNER_POLL_INTERVAL: "${RUNNER_POLL_INTERVAL}"
       RUNNER_MAX_CONCURRENT_SCRAPE: "${RUNNER_MAX_CONCURRENT_SCRAPE}"