* 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.
Additionally, improve HTML pre-processing:
* Ensure a new line between blocks of lyrics text from letras.mus.br.
* Parse a missing last block of lyrics text from lacocinelle.net.
* Parse a missing last block of lyrics text from paroles.net.
* Fix encoding issues with AZLyrics by setting response encoding to
None, allowing `requests` to handle it.
Add explicit checks for lyrics texts fetched from the tested sources.
- Introduced `LyricsPage` class to represent lyrics pages for integrated
tests.
- Configured expected lyrics for each of the URLs that are being
fetched.
- Consolidated integrated tests in a new `TestLyricsSources` class.
- Mocked Google Search API to return the lyrics page under test.
