refactor: 다중 엔진 레지스트리 구조로 전환 + 프론트 엔진 선택 UI

- app/engines: BaseEngine 인터페이스, MeloEngine(인프로세스), 레지스트리
- /api/engines 추가, /api/tts 에 engine 파라미터
- 프론트: 엔진 → 언어 → 목소리 선택, 라이선스/GPU/조절 지원 표기
- MeloTTS 동작 불변

frontend/app.js

@@ -3,6 +3,8 @@ const $ = (id) => document.getElementById(id);
 const els = {
   text: $("text"),
   charCount: $("charCount"),
+  engine: $("engine"),
+  engineInfo: $("engineInfo"),
   language: $("language"),
   speaker: $("speaker"),
   speed: $("speed"),
@@ -19,16 +21,24 @@ const els = {
   deviceChip: $("deviceChip"),
 };
 
-let voices = [];
+let engines = [];
 let lastUrl = null;
 
+function currentEngine() {
+  return engines.find((e) => e.id === els.engine.value);
+}
+function currentLanguage() {
+  const eng = currentEngine();
+  if (!eng) return null;
+  return eng.languages.find((l) => l.code === els.language.value);
+}
+
 function updateCharCount() {
   els.charCount.textContent = els.text.value.length;
 }
 
 function populateSpeakers() {
-  const code = els.language.value;
-  const lang = voices.find((v) => v.code === code);
+  const lang = currentLanguage();
   els.speaker.innerHTML = "";
   if (!lang) return;
   lang.speakers.forEach((s) => {
@@ -39,21 +49,64 @@ function populateSpeakers() {
   });
 }
 
-async function loadVoices() {
+function populateLanguages() {
+  const eng = currentEngine();
+  els.language.innerHTML = "";
+  if (!eng) return;
+  eng.languages.forEach((l) => {
+    const opt = document.createElement("option");
+    opt.value = l.code;
+    opt.textContent = l.label;
+    els.language.appendChild(opt);
+  });
+  populateSpeakers();
+  updateEngineInfo();
+}
+
+function updateEngineInfo() {
+  const eng = currentEngine();
+  if (!eng) {
+    els.engineInfo.textContent = "";
+    return;
+  }
+  const nc = /NC|BY-NC|비상업/i.test(eng.license || "");
+  const gpu = eng.uses_gpu
+    ? "GPU"
+    : '<span class="cpu">CPU</span>';
+  const supports = eng.supports || {};
+  const ctrl = [
+    supports.speed ? "속도" : null,
+    supports.pitch ? "피치" : null,
+  ]
+    .filter(Boolean)
+    .join(" · ");
+  els.engineInfo.innerHTML =
+    `${eng.notes || ""} ` +
+    `<span class="lic${nc ? " nc" : ""}">${eng.license || "?"}</span> ` +
+    `· ${gpu}` +
+    (ctrl ? ` · 조절: ${ctrl}` : "");
+
+  // 지원하지 않는 컨트롤 비활성화
+  els.speed.disabled = supports.speed === false;
+  els.pitch.disabled = supports.pitch === false;
+}
+
+async function loadEngines() {
   try {
-    const res = await fetch("/api/voices");
+    const res = await fetch("/api/engines");
     const data = await res.json();
-    voices = data.languages || [];
-    els.language.innerHTML = "";
-    voices.forEach((v) => {
+    engines = data.engines || [];
+    els.engine.innerHTML = "";
+    engines.forEach((e) => {
       const opt = document.createElement("option");
-      opt.value = v.code;
-      opt.textContent = v.label;
-      els.language.appendChild(opt);
+      opt.value = e.id;
+      opt.textContent = e.label;
+      els.engine.appendChild(opt);
     });
-    populateSpeakers();
+    if (data.default) els.engine.value = data.default;
+    populateLanguages();
   } catch (e) {
-    showError("목소리 목록을 불러오지 못했습니다.");
+    showError("엔진 목록을 불러오지 못했습니다.");
   }
 }
 
@@ -100,6 +153,7 @@ async function generate() {
       headers: { "Content-Type": "application/json" },
       body: JSON.stringify({
         text,
+        engine: els.engine.value,
         language: els.language.value,
         speaker: els.speaker.value,
         speed: parseFloat(els.speed.value),
@@ -129,6 +183,7 @@ async function generate() {
 }
 
 els.text.addEventListener("input", updateCharCount);
+els.engine.addEventListener("change", populateLanguages);
 els.language.addEventListener("change", populateSpeakers);
 els.speed.addEventListener("input", () => {
   els.speedVal.textContent = parseFloat(els.speed.value).toFixed(2) + "x";
@@ -140,5 +195,5 @@ els.pitch.addEventListener("input", () => {
 els.generate.addEventListener("click", generate);
 
 updateCharCount();
-loadVoices();
+loadEngines();
 loadHealth();