feat: detect actual audio format and resolve quality mismatch

format.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func normalizedFormat(format string) string {
+	f := strings.ToLower(strings.TrimPrefix(format, "."))
+	switch f {
+	case "m4a", "mp4", "mmp4":
+		return "m4a"
+	case "mflac", "flac":
+		return "flac"
+	case "mp3":
+		return "mp3"
+	case "ogg", "opus":
+		return "ogg"
+	case "wav":
+		return "wav"
+	case "aac":
+		return "aac"
+	default:
+		return f
+	}
+}
+
+func actualQuality(requested, resolved, format string) string {
+	f := normalizedFormat(format)
+	if f == "flac" {
+		if resolved != "" {
+			return resolved
+		}
+		if requested == "lossless" || requested == "hires" {
+			return requested
+		}
+	}
+	if f == "mp3" {
+		if resolved == "standard" || resolved == "exhigh" || resolved == "sq" {
+			return resolved
+		}
+	}
+	return ""
+}
+
+func renameToActualFormat(local string, format string) (string, error) {
+	ext := normalizedFormat(format)
+	if ext == "" {
+		return local, nil
+	}
+	target := strings.TrimSuffix(local, filepath.Ext(local)) + "." + ext
+	if target == local {
+		return local, nil
+	}
+	if _, err := os.Stat(target); err == nil {
+		if err := os.Remove(local); err != nil {
+			return "", err
+		}
+		return target, nil
+	}
+	if err := os.Rename(local, target); err != nil {
+		return "", fmt.Errorf("rename actual format: %w", err)
+	}
+	return target, nil
+}