refactor: rename Format to Quality, add container Audio/Video sub-pickers

- Rename 'Format' button to 'Quality' in settings (audio_quality/video_quality)
- Add DefaultAudioContainer and DefaultVideoContainer to UserPreferences
- Container settings now has Audio/Video sub-pickers (mp3/m4a/opus vs mp4/mkv/webm)
- Add migration 003 for new container columns
- Update repo layer and playlist flow for new fields
- Update back routing: back_quality, back_container

internal/domain/types.go

@@ -134,11 +134,12 @@ type DownloadJob struct {
 
 // UserPreferences holds per-user default preferences.
 type UserPreferences struct {
-	DefaultMediaType   MediaType
-	DefaultAudioFormat string
-	DefaultVideoFormat string
-	DefaultContainer   string
-	DefaultLanguage    string
+	DefaultMediaType      MediaType
+	DefaultAudioFormat    string
+	DefaultVideoFormat    string
+	DefaultAudioContainer string
+	DefaultVideoContainer string
+	DefaultLanguage       string
 }
 
 // UserQuotas holds the quota usage and limits for a user.