Prepare 2.5.8 release: update the workflow default version_tag and the app _SOULSYNC_BASE_VERSION to 2.5.8, add WHATS_NEW entries for 2.5.8 (fix blank artist pages for Python/git-pull installs, fix premature download completion before post-processing, add disk-backed artwork cache with SQLite, and add pre-download duration tolerancing for strict sources), and update the whats-new fallback to 2.5.8.