Task T9 no-keyvisual evidence

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

Result:
- Empty non-song agenda item adds no playlist entry and no .pro file.
- Song agenda items still export through the normal song .pro path.
- Uploaded agenda slides still export normally; no keyvisual slide is prepended.
- Song agenda items never receive keyvisual fallback slides.

Verification:
- `tests/Feature/KeyVisualFallbackTest.php` covers:
  - no keyvisual → no empty-item .pro entry,
  - song item → only song .pro,
  - uploaded slides → exactly the uploaded slide presentation and no fallback background.
- Full suite after implementation: 541 passed, 2699 assertions.