server-youtube: strip Zone.Identifier ADS on Windows after download

NTFS marks files downloaded over HTTP with a Zone.Identifier alternate
data stream, which SmartScreen/Attachment Manager can use to block
execution of yt-dlp.exe. Remove the ADS best-effort after each
download to reduce one likely cause of "execution verification failed"
in the user-reported failure.

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

src/server/youtube.ts

@@ -91,6 +91,10 @@ async function prepareYtDlp(target: string): Promise<string> {
     await downloadToFile(url, target)
     if (process.platform !== 'win32') {
       await fs.chmod(target, 0o755)
+    } else {
+      // Windows: 인터넷에서 받은 파일에는 NTFS ADS 'Zone.Identifier' 가 붙어
+      // SmartScreen/Attachment Manager 가 실행을 막을 수 있다. 베스트에포트로 제거.
+      try { await fs.unlink(`${target}:Zone.Identifier`) } catch { /* noop */ }
     }
     const probe = await probeVersion(target)
     if (probe.ok) return target