feat: gate download options on real-time yt-dlp video analysis

Options (quality, subtitles) are now derived from a yt-dlp -J probe of
the submitted URL instead of a static list, so users can't pick
combinations the source video doesn't actually support. Submission is
blocked until the probe succeeds.

- New /api/probe endpoint + src/lib/ytdlp-probe.ts, with its own rate
  limiter (rate-limit.ts refactored into a createRateLimiter factory).
- New options: subtitle language selection, clip start/end trim, MP3
  audio quality.
- Fixed buildYtdlpArgs: format=mp3 never triggered audio extraction
  (-x/--audio-format), and quality=best never applied --merge-output-format,
  so the chosen container had no real effect.
- Cross-platform yt-dlp invocation: the bundled bin/yt-dlp is a Python
  zipapp relying on a shebang, which Windows' spawn() can't run
  directly. buildYtdlpCommand() runs it through `python` on Windows and
  execs it directly on Linux/o2switch, where the shebang works natively.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/lib/ytdlp.ts

@@ -8,11 +8,49 @@ export type YtdlpParams = {
   format: string
   quality: string
   subtitles: boolean
+  subtitleLangs: string[] | null
+  clipStart: number | null
+  clipEnd: number | null
+  audioQuality: string | null
   extraArgs: string | null
 }
 
+export function resolveYtdlpBin(): string {
+  const local = path.join(config.BIN_DIR, 'yt-dlp')
+  return existsSync(local) ? local : 'yt-dlp'
+}
+
+export type YtdlpCommand = { command: string; args: string[] }
+
+// The bundled bin/yt-dlp is a Python zipapp relying on a `#!/usr/bin/env python3`
+// shebang: Linux (o2switch prod) execs it natively, but Windows' spawn() has no
+// shebang support and fails with ENOENT, so it must be run through `python` there.
+// A system-wide `yt-dlp` (PATH fallback) already has a proper platform launcher
+// on both OSes and is always run directly.
+export function buildYtdlpCommand(args: string[]): YtdlpCommand {
+  const bin = resolveYtdlpBin()
+  const isBundledScript = bin !== 'yt-dlp'
+
+  if (process.platform === 'win32' && isBundledScript) {
+    return { command: 'python', args: [bin, ...args] }
+  }
+
+  return { command: bin, args }
+}
+
 export function buildYtdlpArgs(params: YtdlpParams): string[] {
-  const { url, uuid, format, quality, subtitles, extraArgs } = params
+  const {
+    url,
+    uuid,
+    format,
+    quality,
+    subtitles,
+    subtitleLangs,
+    clipStart,
+    clipEnd,
+    audioQuality,
+    extraArgs,
+  } = params
   const outputTemplate = path.join(config.STORAGE_PATH, `${uuid}.%(ext)s`)
 
   const args: string[] = ['--no-playlist', '-o', outputTemplate]
@@ -22,13 +60,22 @@ export function buildYtdlpArgs(params: YtdlpParams): string[] {
     args.push('--ffmpeg-location', ffmpegBin)
   }
 
-  if (quality !== 'best') {
-    const height = quality.replace('p', '')
-    args.push('-f', `${format}[height<=?${height}]+bestaudio/best[height<=?${height}]`)
+  if (format === 'mp3') {
+    args.push('-x', '--audio-format', 'mp3')
+    args.push('--audio-quality', audioQuality && audioQuality !== 'best' ? `${audioQuality}K` : '0')
+  } else {
+    const heightFilter = quality !== 'best' ? `[height<=?${quality.replace('p', '')}]` : ''
+    args.push('-f', `bestvideo${heightFilter}+bestaudio/best${heightFilter}`)
+    args.push('--merge-output-format', format)
   }
 
   if (subtitles) {
-    args.push('--write-sub', '--sub-lang', 'fr,en')
+    const langs = subtitleLangs?.length ? subtitleLangs.join(',') : 'fr,en'
+    args.push('--write-subs', '--write-auto-subs', '--sub-lang', langs)
+  }
+
+  if (clipStart != null || clipEnd != null) {
+    args.push('--download-sections', `*${clipStart ?? 0}-${clipEnd ?? ''}`)
   }
 
   if (extraArgs) {