Add is_trakt_group field to PlaylistGroup and update sorting logic

- Introduced `is_trakt_group` field in `PlaylistGroup` struct to differentiate between Trakt and non-Trakt groups.
- Updated relevant parsing functions in `m3u.rs`, `xtream.rs`, and `trakt.rs` to set this field appropriately.
- Enhanced sorting logic in `sort.rs` to prioritize Trakt groups during playlist sorting.
- Ensured backward compatibility by defaulting `is_trakt_group` to false where applicable.

backend/src/processing/parser/xtream.rs

@@ -180,6 +180,7 @@ pub fn parse_xtream(input: &ConfigInput,
                                 xtream_cluster,
                                 title: category.category_name.to_string(),
                                 channels: category.channels.clone(),
+                                is_trakt_group: false,
                             }
                         }).collect()))
                 }