feat: add --backup-converted flag to keep .orig backups

internal/mirror/mirror.go

@@ -44,6 +44,7 @@ type MirrorConfig struct {
 	ConvertLinks    bool // Convert links for local viewing
 	DownloadAssets  bool // Download CSS, JS, images
 	PruneEmpty      bool // Remove empty directories after download
+	BackupConverted bool // Keep .orig backup before link conversion
 	RestrictFiles   bool // Only download files from original host
 	Timeout         time.Duration
 	RateLimiter     *transport.TokenBucket
@@ -642,6 +643,16 @@ func (m *Mirror) convertLinksInFile(filePath string) {
 	converted := m.rewriter.RewriteLinks(doc, filePath)
 
 	if converted > 0 {
+		// Backup original file before link conversion.
+		if m.config.BackupConverted {
+			backupPath := filePath + ".orig"
+			if err := os.WriteFile(backupPath, data, 0644); err != nil {
+				if m.config.Verbose {
+					fmt.Fprintf(os.Stderr, "[error] failed to backup %s: %v\n", filePath, err)
+				}
+			}
+		}
+
 		// Write modified HTML
 		var buf bytes.Buffer
 		if err := html.Render(&buf, doc); err != nil {