Task T8 evidence — no background / excluded slide types

Verified by `Tests\Feature\ProFileExportTest`:
- `test_export_ohne_background_enthaelt_keine_background_actions`
  - Service without resolved background generates successfully
  - exported song slides contain zero BACKGROUND media actions
- `test_information_und_moderation_exports_erhalten_keinen_background`
  - information bundle gets no BACKGROUND media
  - moderation bundle gets no BACKGROUND media
- `test_playlist_export_setzt_background_auf_sermon_folien_und_nicht_auf_informationen`
  - final playlist keeps information slides without BACKGROUND media

Verification commands:
- `ddev exec php artisan test tests/Feature/ProFileExportTest.php` → 13 passed, 76 assertions
- `ddev exec php artisan test` → 537 passed, 2683 assertions