FFmpeg Media 1.0.0-alpha8

Stability + cleanup release. Lifecycle remains experimental for one
more cycle so external test sites can validate the breaking changes
before 1.0.0 stable.

Highlights:
  - Drupal 12 preview compatibility (9 PHPStan + 4 test fixes)
  - drush.services.yml fix — every ffmpeg:* command was hidden
  - Toolbar icon for the FFmpeg Media admin menu entry
  - Permission titles + crossorigin default + UI form gaps closed
  - 4 shared helpers/traits replacing 5 duplicate code clusters
  - 537-line dead-code class removed
  - Live-verified upgrade path from 1.0.0-alpha7 (drush updb runs
    8 update hooks: schema migration, URL relocation cache rebuild,
    permission rename role-grant migration on every existing role)

Breaking changes are documented in CHANGELOG.md under [1.0.0-alpha8]:
  - Admin URLs moved /admin/content/ffmpeg-media/* -> /admin/ffmpeg-media/*
  - 8 permission machine names renamed to <verb> ffmpeg media [...]
  - QueueCommands constructor: 5 services now required