lyrics: add --url option to fetch lyrics from a given URL

[jenish0908]

Description

Fixes #6439.

Adds a --url option to the lyrics command so lyrics can be fetched from a specific URL when the search picks the wrong match (e.g. several LRCLib records for one song):

beet lyrics -f --url https://lrclib.net/api/get/26627014 disco chaos

This picks up from #6548 and follows the review feedback there:

• The option is --url and is read via self.config["url"]. The item is not modified up front; lyrics_url is only stored once lyrics are actually fetched, like for any other match.
• Instead of special-casing LRCLib, each backend now declares which URLs it can handle (URL_PAT / handles_url) and how to fetch them (fetch_url). The URL goes through that backend's normal parsing, so raw HTML is never stored as lyrics:
  • lrclib: https://lrclib.net/api/get/<id> (the source URL beets already prints)
  • genius, tekstowo, musixmatch: their lyrics pages, via the existing scrape()
  • lrcmux: its configured /get?... URL
  • google doesn't claim any URLs since it scrapes arbitrary sites.
• If the URL doesn't belong to any enabled source, the command fails with an error listing the enabled sources. There is no silent fallback to the regular search.
• Since a URL points at a single song, the command errors out if the query matches more than one track.

The existing fetch() methods of the scraping backends and lrcmux now delegate to fetch_url(), so search results and explicit URLs share one code path.

A couple of behaviours worth flagging for review:

• --url does not imply -f, so existing lyrics are kept unless -f is passed (consistent with the other options). Happy to change this if you'd prefer --url to force.
• For LRCLib, the duration ranking is skipped for an explicit URL, but synced lyrics whose last timestamp runs past the track length are still dropped in favour of plain lyrics, same as today.

AI disclosure: I used Claude Code to help implement this and write the tests, following the approach I proposed in the issue. I reviewed and tested all the changes myself.

To Do

• Documentation.
• Changelog.
• Tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.53%. Comparing base (18967aa) to head (6689be1).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7102      +/-   ##
==========================================
+ Coverage   79.26%   79.53%   +0.26%     
==========================================
  Files         149      149              
  Lines       21216    21254      +38     
  Branches     3276     3283       +7     
==========================================
+ Hits        16817    16904      +87     
+ Misses       3585     3537      -48     
+ Partials      814      813       -1     

Files with missing lines	Coverage Δ
beetsplug/lyrics.py	92.45% <100.00%> (+9.24%)	⬆️

... and 2 files with indirect coverage changes

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