Task T9 keyvisual fallback evidence

Scenario: Service has key_visual_filename and a visible non-song agenda item with no uploaded/special slides.

Result:
- PlaylistExportService creates an embedded .pro for the agenda item title.
- The generated .pro contains exactly one image-only slide with background media.
- Background media URL is Storage::disk('public')->path($keyvisual).
- Slide rows are not created; Slide::count() remains unchanged.

Verification:
- RED before implementation: `ddev exec php artisan test tests/Feature/KeyVisualFallbackTest.php` failed with `Keine Songs mit Inhalt zum Exportieren gefunden.` for empty non-song agenda item.
- GREEN after implementation: `ddev exec php artisan test tests/Feature/KeyVisualFallbackTest.php` → 4 passed, 16 assertions.
- Full suite: `ddev exec php artisan test` → 541 passed, 2699 assertions.
- Pint: `ddev exec ./vendor/bin/pint app/Services/PlaylistExportService.php tests/Feature/KeyVisualFallbackTest.php` → PASS, 2 files.
