feat: use Edge TTS (Korean Hyunsu voice @ +45%) as the default voice

The user chose Microsoft Edge TTS, voice ko-KR-HyunsuMultilingualNeural at rate
+45% (~1.45x), as the natural Korean voice. Wire it into the bridge and make it
the default engine.

- bridge/server.py: _edge_synthesize() calls edge-tts and transcodes the MP3 to
  PCM16 mono WAV with the system ffmpeg (temp file for a correct header);
  TTS_ENGINE default -> edge; EDGE_TTS_VOICE / EDGE_TTS_RATE env-driven
- requirements-bridge.txt: add edge-tts (lightweight; httpx)
- compose/.env.example/README: TTS_ENGINE=edge + EDGE_TTS_* knobs; note the
  online/privacy trade-off (reply text is sent to Microsoft, needs internet)
- drop the now-unused MeloTTS build layer (Dockerfile) and melo-worker
  (supervisord) — edge synthesises in-process, no model/worker baked, slimmer
  and faster image; settings UI engine list -> edge/piper, restart only bridge

Verified on host: edge-tts -> ffmpeg yields a valid 16-bit mono 24kHz WAV;
envsubst renders tts_engine=edge; docker build --check + 26 tests pass.

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:edge,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):
@@ -114,15 +106,15 @@ 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 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. (Edge TTS has no worker.)
     try:
         subprocess.Popen(
-            ["sh", "-c", "sleep 1; supervisorctl restart melo-worker bridge"],
+            ["sh", "-c", "sleep 1; supervisorctl restart bridge"],
             start_new_session=True,
         )
-        return "1초 후 브리지/TTS 워커가 재시작되어 반영됩니다."
+        return "1초 후 브리지가 재시작되어 반영됩니다."
     except Exception as e:  # pragma: no cover
         return str(e)