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>

bridge/settings_web.py

@@ -22,8 +22,7 @@ from typing import Any, Dict
 FIELDS = [
     ("ollama_chat_model", "LLM 모델", "model"),
     ("whisper_model", "STT(Whisper) 모델", "select:tiny,base,small,medium,large,large-v3"),
-    ("tts_engine", "TTS 엔진", "select:melo,piper"),
-    ("melo_speed", "TTS 속도 (MeloTTS)", "number:0.5:2.5:0.1"),
+    ("tts_engine", "TTS 엔진", "select:xtts,piper"),
     ("output_language", "출력 언어 (비우면 사용자 언어)", "text"),
     ("llm_thinking_enabled", "LLM 사고(thinking) 모드", "bool"),
     ("agentic_max_turns", "에이전트 최대 반복", "number:1:12:1"),
@@ -54,9 +53,7 @@ def _current() -> Dict[str, Any]:
     cfg = _read_config()
     out: Dict[str, Any] = {}
     for k in _KEYS:
-        if k == "melo_speed":
-            out[k] = cfg.get("melo_speed", os.environ.get("MELO_SPEED", "1.5"))
-        elif k == "output_language":
+        if k == "output_language":
             out[k] = cfg.get("output_language", os.environ.get("OUTPUT_LANGUAGE", ""))
         else:
             out[k] = cfg.get(k, "")
@@ -78,12 +75,7 @@ def _coerce(updates: Dict[str, Any]) -> Dict[str, Any]:
     for k, v in updates.items():
         if k not in _KEYS:
             continue
-        if k == "melo_speed":
-            try:
-                v = float(v)
-            except (TypeError, ValueError):
-                continue
-        elif k == "agentic_max_turns":
+        if k == "agentic_max_turns":
             try:
                 v = int(v)
             except (TypeError, ValueError):
@@ -114,12 +106,12 @@ def _save(updates: Dict[str, Any]) -> None:
 
 
 def _apply() -> str:
-    # Restart melo + bridge AFTER this response is sent. Detached (new session)
-    # so the bridge being killed mid-restart doesn't drop the restart itself,
-    # and the HTTP client still receives this response.
+    # Restart the TTS worker + bridge AFTER this response is sent. Detached (new
+    # session) so the bridge being killed mid-restart doesn't drop the restart
+    # itself, and the HTTP client still receives this response.
     try:
         subprocess.Popen(
-            ["sh", "-c", "sleep 1; supervisorctl restart melo-worker bridge"],
+            ["sh", "-c", "sleep 1; supervisorctl restart xtts-worker bridge"],
             start_new_session=True,
         )
         return "1초 후 브리지/TTS 워커가 재시작되어 반영됩니다."