server-youtube: fast-path reuse for cached zipapp

ensureYtDlp() and prepareYtDlp() now check the on-disk yt-dlp_zipapp
before re-running the native-fail -> network-download path. On Linux
servers where the native binary always fails verification (glibc/musl/
arch mismatch), every previous request was re-downloading both the
native (~33MB) and the zipapp (~3MB). With the fast path, after the
first successful zipapp install all subsequent requests short-circuit
to the cached zipapp.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/server/youtube.ts

@@ -69,6 +69,14 @@ export async function ensureYtDlp(): Promise<string> {
     const probe = await probeVersion(target)
     if (probe.ok) return target
   }
+  // Fast path: 네이티브가 안 도는 환경에서 이전에 받아둔 zipapp 이 살아있으면 그걸 재사용
+  if (process.platform !== 'win32') {
+    const zipappPath = getYtDlpZipappPath()
+    if (await fileExists(zipappPath)) {
+      const probe = await probeVersion(zipappPath)
+      if (probe.ok) return zipappPath
+    }
+  }
   if (installPromise) return installPromise
   installPromise = (async () => {
     try {
@@ -83,13 +91,23 @@ export async function ensureYtDlp(): Promise<string> {
 async function prepareYtDlp(target: string): Promise<string> {
   const diagnostics: string[] = []
 
-  // 1. 기존 파일이 있으면 우선 그걸로 시도
+  // 1a. 기존 네이티브 파일이 있으면 우선 그걸로 시도
   if (await fileExists(target)) {
     const probe = await probeVersion(target)
     if (probe.ok) return target
     diagnostics.push(`기존 ${path.basename(target)} 검증 실패: ${probe.detail}`)
   }
 
+  // 1b. (POSIX) 기존 zipapp 이 있으면 재다운로드 전에 먼저 시도
+  if (process.platform !== 'win32') {
+    const existingZipapp = getYtDlpZipappPath()
+    if (await fileExists(existingZipapp)) {
+      const probe = await probeVersion(existingZipapp)
+      if (probe.ok) return existingZipapp
+      diagnostics.push(`기존 yt-dlp_zipapp 검증 실패: ${probe.detail}`)
+    }
+  }
+
   // 2. PATH 에 yt-dlp(.exe) 가 시스템 전역으로 설치돼 있으면 그걸 사용
   const pathCmd = process.platform === 'win32' ? 'yt-dlp.exe' : 'yt-dlp'
   const pathProbe = await probeVersion(pathCmd)