Auto-install yt-dlp into %appdata%/.mc_custom on first use

The previous flow required the operator to manually install yt-dlp on the
server. Now the server downloads the correct binary for the current OS/arch
from GitHub Releases (latest) the first time a playlist fetch is requested,
caches it under %appdata%/.mc_custom (resolved per-OS), chmod +x's it on
POSIX, and skips the download on every subsequent call.

- shared/paths.ts: add getAppDataDir() + getMcCustomDir() that map %appdata%
  to ~/.config (Linux), ~/Library/Application Support (macOS), or
  process.env.APPDATA (Windows) so the path is OS-agnostic.
- server/youtube.ts: replace the old "probe PATH and bail" probeYtDlp with
  ensureYtDlp(): picks the right GitHub asset name per platform.arch
  (yt-dlp.exe / yt-dlp_macos / yt-dlp_linux / yt-dlp_linux_aarch64 /
  yt-dlp_linux_armv7l), downloads it with redirect-following https.get to
  the install path, chmods +x, then verifies with `--version` before
  reporting success. Uses an installPromise singleton so concurrent
  requests don't race the download. On any failure it cleans up the
  partial file and throws YtDlpUnavailableError with a real reason.
- docs/yt-dlp-setup.md: note that auto-install is now the default; manual
  install is only for environments where the auto-download fails.

Smoke-tested on this Linux x64 box: first call downloads to
~/.config/.mc_custom/yt-dlp_linux and reports `2026.03.17`; second call
takes ~700ms (just the version probe) and reuses the cached file.

src/server/youtube.ts

@@ -1,4 +1,9 @@
 import { spawn } from 'node:child_process'
+import { promises as fs, createWriteStream, constants as fsConst } from 'node:fs'
+import path from 'node:path'
+import https from 'node:https'
+import http from 'node:http'
+import { getMcCustomDir } from '../shared/paths'
 
 export interface YtPlaylistEntry {
   id: string
@@ -9,34 +14,138 @@ export interface YtPlaylistEntry {
 }
 
 export class YtDlpUnavailableError extends Error {
-  constructor() {
-    super('서버에 yt-dlp가 설치돼 있지 않습니다. (수동 입력으로 진행)')
+  constructor(message?: string) {
+    super(message || 'yt-dlp 를 준비하지 못했습니다. (수동 입력으로 진행)')
   }
 }
 
+/** 현재 OS/아키텍처에서 GitHub Releases 가 제공하는 yt-dlp 파일 이름. */
+function getYtDlpAssetName(): string {
+  if (process.platform === 'win32') return 'yt-dlp.exe'
+  if (process.platform === 'darwin') return 'yt-dlp_macos'
+  if (process.platform === 'linux') {
+    if (process.arch === 'arm64') return 'yt-dlp_linux_aarch64'
+    if (process.arch === 'arm') return 'yt-dlp_linux_armv7l'
+    return 'yt-dlp_linux'
+  }
+  return 'yt-dlp' // 그 외 OS: 순수 파이썬 zipapp. python3 가 PATH 에 있어야 동작
+}
+
+/** 로컬 설치 경로: %appdata%/.mc_custom/<asset> */
+export function getYtDlpInstallPath(): string {
+  return path.join(getMcCustomDir(), getYtDlpAssetName())
+}
+
+/** 한 번에 한 다운로드만 진행하도록 락 (서버 동시 요청 보호). */
+let installPromise: Promise<string> | null = null
+
 /**
- * yt-dlp 가 시스템에 있는지와 그 경로를 빠르게 확인.
- * 없으면 YtDlpUnavailableError.
+ * %appdata%/.mc_custom/ 에 yt-dlp 가 준비됐는지 확인하고, 없으면 GitHub Releases 에서
+ * 현재 OS/아키텍처용 바이너리를 자동으로 받아 설치한다. 성공 시 실행 경로 반환.
  */
-async function probeYtDlp(): Promise<string> {
+export async function ensureYtDlp(): Promise<string> {
+  const target = getYtDlpInstallPath()
+  // 이미 설치돼 있고 실행 가능하면 그대로 사용
+  if (await canExecute(target)) return target
+  if (installPromise) return installPromise
+  installPromise = (async () => {
+    try {
+      const dir = getMcCustomDir()
+      await fs.mkdir(dir, { recursive: true })
+      const asset = getYtDlpAssetName()
+      const url = `https://github.com/yt-dlp/yt-dlp/releases/latest/download/${asset}`
+      await downloadToFile(url, target)
+      // POSIX 계열은 실행 권한 부여
+      if (process.platform !== 'win32') {
+        await fs.chmod(target, 0o755)
+      }
+      // 검증
+      const okVersion = await probeVersion(target)
+      if (!okVersion) {
+        throw new YtDlpUnavailableError('yt-dlp 다운로드는 됐지만 실행 검증에 실패했습니다.')
+      }
+      return target
+    } catch (err) {
+      // 실패 흔적(부분 다운로드) 삭제
+      try { await fs.unlink(target) } catch { /* noop */ }
+      throw err instanceof YtDlpUnavailableError
+        ? err
+        : new YtDlpUnavailableError(
+            'yt-dlp 자동 설치에 실패했습니다: ' + (err instanceof Error ? err.message : String(err))
+          )
+    } finally {
+      installPromise = null
+    }
+  })()
+  return installPromise
+}
+
+async function canExecute(filePath: string): Promise<boolean> {
+  try {
+    await fs.access(filePath, fsConst.F_OK)
+  } catch {
+    return false
+  }
+  // POSIX 면 X 비트도 확인
+  if (process.platform !== 'win32') {
+    try {
+      await fs.access(filePath, fsConst.X_OK)
+    } catch {
+      return false
+    }
+  }
+  // 실제로 --version 으로 한 번 더 확인
+  return probeVersion(filePath)
+}
+
+function probeVersion(bin: string): Promise<boolean> {
+  return new Promise((resolve) => {
+    const child = spawn(bin, ['--version'], { stdio: ['ignore', 'pipe', 'pipe'] })
+    let ok = false
+    child.stdout.on('data', () => { ok = true })
+    child.on('error', () => resolve(false))
+    child.on('close', (code) => resolve(ok && code === 0))
+  })
+}
+
+/** GitHub Releases 의 latest URL 은 302 리다이렉트를 사용하므로 따라가며 받음. */
+function downloadToFile(url: string, dest: string, redirects = 0): Promise<void> {
   return new Promise((resolve, reject) => {
-    const probe = spawn('yt-dlp', ['--version'], { stdio: ['ignore', 'pipe', 'pipe'] })
-    let stderr = ''
-    probe.stderr.on('data', (chunk: Buffer) => (stderr += chunk.toString('utf8')))
-    probe.on('error', () => reject(new YtDlpUnavailableError()))
-    probe.on('close', (code) => {
-      if (code === 0) resolve('yt-dlp')
-      else reject(new Error(`yt-dlp 실행 실패 (code=${code}): ${stderr}`))
+    if (redirects > 8) {
+      reject(new Error('redirect 가 너무 많습니다.'))
+      return
+    }
+    const lib = url.startsWith('https://') ? https : http
+    const req = lib.get(url, {
+      headers: { 'user-agent': 'mc-music-quiz-launcher' }
+    }, (res) => {
+      const code = res.statusCode || 0
+      if (code >= 300 && code < 400 && res.headers.location) {
+        res.resume()
+        downloadToFile(res.headers.location, dest, redirects + 1).then(resolve, reject)
+        return
+      }
+      if (code !== 200) {
+        res.resume()
+        reject(new Error(`HTTP ${code} (${url})`))
+        return
+      }
+      const out = createWriteStream(dest)
+      res.pipe(out)
+      out.on('finish', () => out.close((err) => err ? reject(err) : resolve()))
+      out.on('error', reject)
+      res.on('error', reject)
     })
+    req.on('error', reject)
   })
 }
 
 /**
- * 플레이리스트 URL 을 yt-dlp 로 펼쳐 각 영상의 메타데이터를 가져온다.
+ * 플레이리스트 URL 을 펼쳐 각 영상의 메타데이터를 가져온다.
  * `--flat-playlist --dump-json` 출력은 한 줄당 한 JSON.
  */
 export async function fetchPlaylistEntries(url: string): Promise<YtPlaylistEntry[]> {
-  const bin = await probeYtDlp()
+  const bin = await ensureYtDlp()
   return new Promise((resolve, reject) => {
     const child = spawn(bin, [
       '--flat-playlist',