refactor(model): support string type in JSON scan methods

Enhance Scan methods in StringArray, ExtraData, MessageSegments, and
SystemNotificationExtra to accept both []byte and string types from database.
Uses type switch for more flexible type handling.

internal/model/system_notification.go

@@ -79,11 +79,16 @@ func (e *SystemNotificationExtra) Scan(value any) error {
 	if value == nil {
 		return nil
 	}
-	bytes, ok := value.([]byte)
-	if !ok {
-		return errors.New("type assertion to []byte failed")
+	var data []byte
+	switch v := value.(type) {
+	case []byte:
+		data = v
+	case string:
+		data = []byte(v)
+	default:
+		return errors.New("type assertion to []byte or string failed")
 	}
-	return json.Unmarshal(bytes, e)
+	return json.Unmarshal(data, e)
 }
 
 // SystemNotification 系统通知（独立表，与消息完全分离）