server-youtube: diagnostic detail + PATH fallback when bundled yt-dlp won't run

probeVersion() now captures stderr/exit-code/signal/spawn-error instead of
returning a bare boolean, and ensureYtDlp() tries the bundled binary first,
falls back to `yt-dlp(.exe)` on PATH if the bundled one won't execute (AV
block, missing libc symbol, broken download), and only then re-downloads.
The final user-facing error includes the per-attempt diagnostics so we can
actually see WHY verification failed instead of the opaque
"yt-dlp 다운로드는 됐지만 실행 검증에 실패했습니다." message.

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

locales/server/ko-kr.json

@@ -220,6 +220,7 @@
   "youtube": {
     "ytdlpUnavailable": "yt-dlp 를 준비하지 못했습니다. (수동 입력으로 진행)",
     "ytdlpVerifyFailed": "yt-dlp 다운로드는 됐지만 실행 검증에 실패했습니다.",
+    "ytdlpVerifyFailedDetail": "yt-dlp 를 사용할 수 없습니다. 시도한 경로 진단: {{detail}}",
     "ytdlpInstallFailed": "yt-dlp 자동 설치에 실패했습니다: {{message}}",
     "ytdlpVideoFailed": "yt-dlp 영상 조회 실패 (code={{code}}): {{detail}}",
     "ytdlpPlaylistFailed": "yt-dlp 플레이리스트 조회 실패 (code={{code}}): {{detail}}",