feat: replace MeloTTS with Coqui XTTS-v2 natural Korean voice

MeloTTS's single Korean speaker sounded non-native ("foreign accent"). Swap it
for Coqui XTTS-v2 with the built-in female studio speaker "Ana Florence"
(language ko), the natural voice used in earlier local runs.

- bridge/xtts_worker.py: new warm HTTP worker (own /opt/xtts venv), same
  /synth + /health contract and PCM16 output as the old melo worker
- docker/setup-xtts.sh: builds the venv with cu128 torch (Blackwell) + Coqui
  TTS and bakes the XTTS-v2 model offline. Pins transformers>=4.57,<5 (5.x
  removed isin_mps_friendly, breaking XTTS) and installs the [codec] extra
  (torch>=2.9 needs torchcodec) — both verified by a real host synth
- Dockerfile: replace the melo build layer with the xtts layer
- supervisord.conf: melo-worker -> xtts-worker, env passthrough for
  XTTS_DEVICE/SPEAKER/LANGUAGE (always set via compose defaults)
- bridge/server.py: default TTS_ENGINE=xtts, route to the xtts worker, generic
  worker-synth helper, neural-only fallback flag (XTTS_FALLBACK_PIPER)
- settings UI: engine dropdown xtts/piper, drop the dead melo_speed field, fix
  the supervisorctl restart target to xtts-worker
- compose/.env.example/README: XTTS_* vars, speaker/language knobs, remove melo
- remove bridge/melo_worker.py and docker/setup-melo.sh
- tests: xtts treated as multilingual (not English-only)

Verified on host: coqui-tts loads XTTS-v2 and synthesises Korean as
"Ana Florence" to a 16-bit mono 24kHz WAV.

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

docker/supervisord.conf

@@ -49,25 +49,22 @@ 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=/app/docker/run-if-role.sh full,bot /opt/melo/bin/python /app/bridge/melo_worker.py
+[program:xtts-worker]
+; Warm Coqui XTTS-v2 Korean voice (natural female "Ana Florence") in its own
+; py3.11 venv. The bridge's synthesize() POSTs here; if this is down the bridge
+; falls back to Piper (English) only when XTTS_FALLBACK_PIPER=1.
+command=/app/docker/run-if-role.sh full,bot /opt/xtts/bin/python /app/bridge/xtts_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.
-; MELO_DEVICE and MELO_SPEED inherit from the container env (compose sets both
-; with defaults: cuda / 1.5) so the worker runs MeloTTS on the GPU at the
-; configured rate. supervisord interpolates %(ENV_x)s from its own environment,
-; which is the container's — so MELO_SPEED must always be set in the env
-; (compose guarantees it) or this expansion fails at startup. Hardcoding 1.5
-; here previously shadowed the .env value, so lowering MELO_SPEED had no effect.
-environment=MELO_LANGUAGE="KR",MELO_SPEED="%(ENV_MELO_SPEED)s",MELO_DEVICE="%(ENV_MELO_DEVICE)s",MELO_WORKER_HOST="127.0.0.1",MELO_WORKER_PORT="8770",HF_HOME="/opt/melo-cache",HF_HUB_OFFLINE="1",TRANSFORMERS_OFFLINE="1"
+; TTS_HOME points at the dedicated, image-baked XTTS cache (warmed in
+; setup-xtts.sh). The brain's whisper_cache volume is mounted over
+; /root/.cache, so a dedicated non-volume cache dir avoids the baked model being
+; shadowed and re-downloaded (which would fail if the host is offline).
+; XTTS_DEVICE / XTTS_SPEAKER / XTTS_LANGUAGE inherit from the container env
+; (compose sets them with defaults: cuda / "Ana Florence" / ko). supervisord
+; interpolates %(ENV_x)s from its own environment, which is the container's — so
+; these must always be set in the env (compose guarantees it) or this expansion
+; fails at startup. COQUI_TOS_AGREED accepts the non-commercial XTTS license.
+environment=XTTS_DEVICE="%(ENV_XTTS_DEVICE)s",XTTS_SPEAKER="%(ENV_XTTS_SPEAKER)s",XTTS_LANGUAGE="%(ENV_XTTS_LANGUAGE)s",XTTS_WORKER_HOST="127.0.0.1",XTTS_WORKER_PORT="8771",TTS_HOME="/opt/xtts-cache",COQUI_TOS_AGREED="1"
 priority=280
 autorestart=true
 stdout_logfile=/dev/stdout