v0.6.2

Covers for Spotify tracks, a second try, and Fill in missing covers

- Spotify: the playlist embed page has no images per track, so track()
  also reads each new track's own embed page (embed/track/<id>, 0.5 s
  apart): the 640 px album cover and the release date. When that page
  fails, the playlist details still do. Spotify's image hosts (scdn.co,
  spotifycdn.com) may now be downloaded for the tags.
- Engine.cover_for: a track without an artwork link, or whose link no
  longer loads, is looked up again at its source (the new link is
  stored); a failed cover download is tried once more.
- Settings, Names and tags: "Fill in missing covers" gives every done
  file without a readable cover one (tagging.set_cover writes only the
  picture), with progress in the state API (covers_state).
- An existing FLAC picture is now measured from the image itself, so a
  broken one no longer blocks a good cover.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QppgauJ4gJFJ26CJDxSAw4