refactor(video-pipeline): 用 music-metadata 替换 ffprobe 测量音频时长

ffprobe 在 Mac 上有路径/权限兼容性问题，改用纯 JS 的 music-metadata
实现跨平台音频时长测量，零外部依赖。

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

.claude/skills/video-from-script/scripts/lib/capcut-api.js

@@ -8,7 +8,7 @@
 const axios = require('axios')
 const path = require('path')
 const fs = require('fs')
-const { execFile } = require('child_process')
+const mm = require('music-metadata')
 
 const US = 1_000_000
 
@@ -78,17 +78,14 @@ function getResolution(format) {
   return map[format] || map['9:16']
 }
 
-function getAudioDurationSec(filePath) {
-  return new Promise((resolve) => {
-    execFile('ffprobe', [
-      '-v', 'quiet', '-show_entries', 'format=duration',
-      '-of', 'csv=p=0', filePath
-    ], (err, stdout) => {
-      if (err) { resolve(null); return }
-      const dur = parseFloat(stdout.trim())
-      resolve(dur > 0 ? dur : null)
-    })
-  })
+async function getAudioDurationSec(filePath) {
+  try {
+    const metadata = await mm.parseFile(filePath)
+    const dur = metadata.format.duration
+    return dur > 0 ? dur : null
+  } catch (_) {
+    return null
+  }
 }
 
 module.exports = { US, getConfig, BASE_URL, api, parseArgs, getResolution, getAudioDurationSec }