fix error and new docs

scripts/ttrpg-release/prepare-release.ps1

@@ -45,6 +45,44 @@ function Write-Fail([string]$text) {
   Write-Host "  [!!] $text" -ForegroundColor Red
 }
 
+$FfmpegStaticMirror = 'https://cdn.npmmirror.com/binaries/ffmpeg-static'
+
+function Test-IsNpmCi([string[]]$NpmArgs) {
+  return ($NpmArgs.Count -eq 1 -and $NpmArgs[0] -eq 'ci')
+}
+
+function Invoke-NpmRaw {
+  param(
+    [string[]]$NpmArgs,
+    [string]$WorkingDirectory
+  )
+  Push-Location $WorkingDirectory
+  try {
+    & npm @NpmArgs
+    return $LASTEXITCODE
+  } finally {
+    Pop-Location
+  }
+}
+
+function Invoke-NpmCiWithFfmpegMirror {
+  param(
+    [string]$WorkingDirectory
+  )
+  $prevMirror = $env:FFMPEG_BINARIES_URL
+  $env:FFMPEG_BINARIES_URL = $FfmpegStaticMirror
+  try {
+    Write-Host "  > npm ci (retry with FFMPEG_BINARIES_URL=$FfmpegStaticMirror)"
+    return (Invoke-NpmRaw -NpmArgs @('ci') -WorkingDirectory $WorkingDirectory)
+  } finally {
+    if ($null -eq $prevMirror) {
+      Remove-Item Env:\FFMPEG_BINARIES_URL -ErrorAction SilentlyContinue
+    } else {
+      $env:FFMPEG_BINARIES_URL = $prevMirror
+    }
+  }
+}
+
 function Invoke-Npm {
   param(
     [string]$Label,
@@ -52,14 +90,13 @@ function Invoke-Npm {
     [string]$WorkingDirectory
   )
   Write-Host "  > $Label"
-  Push-Location $WorkingDirectory
-  try {
-    & npm @NpmArgs
-    if ($LASTEXITCODE -ne 0) {
-      throw "$Label failed (exit $LASTEXITCODE)"
-    }
-  } finally {
-    Pop-Location
+  $exitCode = Invoke-NpmRaw -NpmArgs $NpmArgs -WorkingDirectory $WorkingDirectory
+  if ($exitCode -ne 0 -and (Test-IsNpmCi $NpmArgs)) {
+    Write-Host "  [--] npm ci failed (exit $exitCode). Retrying via ffmpeg-static mirror..." -ForegroundColor Yellow
+    $exitCode = Invoke-NpmCiWithFfmpegMirror $WorkingDirectory
+  }
+  if ($exitCode -ne 0) {
+    throw "$Label failed (exit $exitCode)"
   }
 }