* Introduce a `Lyrics` dataclass to carry text, source URL, and language
metadata through fetch, translation, and storage paths.
* Return `Lyrics` from backends and plugin lookup methods instead of raw
tuples/strings.
* Store backend name in `lyrics_source` derived from fetched URL root
domain.
* Simplify translator flow to operate on `Lyrics`, reuse line splitting,
append translations in-place, and record translation language
metadata.