feat: add total duration for playlist

app/Models/MusicPlaylist.php

@@ -71,4 +71,10 @@ public function getTotalDurationAttribute()
     {
         return $this->musics()->where('music.is_active', true)->sum('music.duration');
     }
+
+    // Total play time of the playlist = sum of its active musics' durations.
+    public function getDurationAttribute()
+    {
+        return $this->total_duration;
+    }
 }