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>

.env.example

@@ -34,18 +34,23 @@ WHISPER_DEVICE=cuda
 WHISPER_COMPUTE_TYPE=float16
 # Optional explicit Piper voice model (.onnx). If empty, the jarvis default is used.
 TTS_PIPER_MODEL_PATH=
-# TTS engine: "melo" (default) uses the MeloTTS Korean voice served by the warm
-# melo-worker (Korean speaker, speed 1.5). Set to "piper" to use Piper directly.
-TTS_ENGINE=melo
-# Melo-only by default: if MeloTTS synthesis fails the bridge returns no audio
+# TTS engine: "xtts" (default) uses the Coqui XTTS-v2 natural Korean voice
+# served by the warm xtts-worker. Set to "piper" to use the English Piper voice
+# directly. (MeloTTS was removed; "melo" only works with an out-of-band worker.)
+TTS_ENGINE=xtts
+# XTTS-v2 voice settings. Speaker is any built-in studio voice; "Ana Florence"
+# is a natural female voice. Language is the synthesis language (ko = Korean).
+XTTS_SPEAKER=Ana Florence
+XTTS_LANGUAGE=ko
+XTTS_DEVICE=cuda
+# Where the bridge reaches the in-container XTTS worker, and how long it waits
+# for a synthesis (XTTS is slower than Melo: ~1-2s/sentence on GPU).
+XTTS_WORKER_URL=http://127.0.0.1:8771
+XTTS_TIMEOUT=30
+# Neural-only by default: if XTTS synthesis fails the bridge returns no audio
 # rather than speaking Korean through the English Piper voice (which mangles it).
 # Set to 1 only if you explicitly want the Piper fallback.
-MELO_FALLBACK_PIPER=0
-# Where the bridge reaches the in-container MeloTTS worker, and how long it
-# waits for a synthesis. Speaking rate is set on the worker via MELO_SPEED.
-MELO_WORKER_URL=http://127.0.0.1:8770
-MELO_TIMEOUT=30
-MELO_SPEED=1.5
+XTTS_FALLBACK_PIPER=0
 
 # ---------------------------------------------------------------------------
 # Jarvis brain (Ollama-backed). In Docker these populate the rendered
@@ -226,7 +231,7 @@ COMPOSE_FILE=docker-compose.yml:docker-compose.gpu-linux.yml
 # OLLAMA_CHAT_MODEL=qwen2.5:7b   # quality (needs ~5GB VRAM + whisper small)
 # OLLAMA_CHAT_MODEL=qwen2.5:3b   # speed (fits easily, faster on 8GB GPUs)
 # WHISPER_MODEL=small            # small frees VRAM for a bigger LLM; medium=more accurate
-# MELO_DEVICE=cuda               # cpu if no GPU on the bot host
+# XTTS_DEVICE=cuda               # cpu if no GPU on the bot host (XTTS is slow on CPU)
 
 # --- Settings web UI (http://localhost:8765/settings on the bot host) ---
 # To reach it, expose the bridge to the host loopback: