feat(tts): add MeloTTS Korean voice via warm worker with offline-baked cache

Adds a dedicated MeloTTS Korean voice (speed 1.5) as the primary TTS engine,
served by a long-lived in-container worker so each Discord turn pays only
inference cost, not model-load cost.

- bridge/melo_worker.py: tiny HTTP service in its own /opt/melo py3.11 venv,
  keeps the KR model warm, returns PCM16 WAV on POST /synth.
- bridge/server.py: synthesize() routes to the melo worker first; Piper stays
  as an opt-in fallback (MELO_FALLBACK_PIPER, default off so Korean is never
  mangled through the English voice). /health reports tts_engine.
- docker/setup-melo.sh: builds the isolated venv (pinned torch 2.12.0 /
  torchaudio 2.11.0 CPU, MeloTTS pinned to a commit for reproducible rebuilds),
  pre-fetches mecab-ko, and warms a dedicated HF cache (/opt/melo-cache) with a
  real KR synth so all BERT + KR checkpoint assets are baked into the image.
- docker/supervisord.conf: runs melo-worker before the bridge with
  HF_HOME=/opt/melo-cache (the whisper_cache volume shadows the default HF
  cache) plus HF_HUB_OFFLINE/TRANSFORMERS_OFFLINE so it reads the baked cache
  and never retries the network on load.
- Dockerfile/.env.example: wire the melo build layer and config knobs.

Verified: offline synth passes with --network none and the prod volume mounted;
prod container recreated, all supervisord services up, bot logged in, and an
end-to-end /tts call returns a 44.1kHz mono PCM16 WAV.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker/supervisord.conf

@@ -49,6 +49,26 @@ stdout_logfile_maxbytes=0
 stderr_logfile=/dev/stderr
 stderr_logfile_maxbytes=0
 
+[program:melo-worker]
+; Warm MeloTTS Korean voice (speed 1.5) in its own py3.11 venv. The bridge's
+; synthesize() POSTs here; if this is down the bridge falls back to Piper.
+command=/opt/melo/bin/python /app/bridge/melo_worker.py
+directory=/app
+; HF_HOME points at the dedicated, image-baked melo cache (warmed in
+; setup-melo.sh). The brain's whisper_cache volume is mounted over
+; /root/.cache/huggingface, so without this the pre-cached BERT + KR checkpoint
+; would be shadowed and re-downloaded (and would fail if the host is offline).
+; HF_HUB_OFFLINE/TRANSFORMERS_OFFLINE force pure-cache reads: the pinned old
+; transformers/huggingface_hub otherwise retry the network on every load and
+; error out instead of falling back to the (complete) baked cache.
+environment=MELO_LANGUAGE="KR",MELO_SPEED="1.5",MELO_DEVICE="cpu",MELO_WORKER_HOST="127.0.0.1",MELO_WORKER_PORT="8770",HF_HOME="/opt/melo-cache",HF_HUB_OFFLINE="1",TRANSFORMERS_OFFLINE="1"
+priority=280
+autorestart=true
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+
 [program:bridge]
 command=/opt/venv/bin/python -m bridge.server
 directory=/app