vf-video-teaser (accessibility fixes)

[catortora]

This is a sub-task of #2411 (vf-video-teaser usage documentation).

Description

vf-video-teaser renders images that link to videos hosted elsewhere; it plays no video. #2131 identified several accessibility issues that hold as long as the component exists: its published examples model accessibility failures authors could copy, its heading level is fixed, and its whole-item link needs focus verification. This ticket addresses them.

Reclassification of the component (card / summary / standalone) is deferred to the broader deprecation/modernisation discussion. These fixes apply while the component exists and do not pre-empt that decision.

Phase 1: Evaluate

Heading level: how should it be made configurable?

• Decide the author-facing property for the heading level, and its documented default.

Phase 2: Implement

• Fix the documented examples. The published examples ship alt="" and href="JavaScript:Void(0);". Replace the placeholders with meaningful alternative text and valid destinations, since examples are what authors copy (WCAG 1.1.1 A, 2.4.4 A).
• Support the alt-text decision in the template. Ensure the component lets an author supply meaningful alternative text or an empty alt for a decorative thumbnail, consistent with the guidance in #2411 (WCAG 1.1.1 A).
• Configurable heading level. Add the heading-level property per the Phase 1 decision, with the documented default (WCAG 1.3.1 A).
• Verify keyboard focus visibility. The whole item is made clickable via a ::after overlay on the link; verify the focus state is visible on keyboard navigation and fix if not (WCAG 2.4.7 AA).

Out of scope

• Reclassifying, renaming or replacing the component: deferred to the deprecation/modernisation discussion.
• Changing what the component renders (images and links) or its click behaviour.

Success criteria

• Phase 1: heading-level property design confirmed
• Documented examples use meaningful alt text and valid link destinations
• The template supports both meaningful and decorative (alt="") poster images
• The heading level is configurable, with a documented default
• Keyboard focus is visible on the whole-item link

Definition of Done

• Phase 1 evaluation recorded (heading-level design)
• Examples, alt-text handling, heading level and focus state updated
• Changelog entry added
• #2411 informed so the documentation reflects the changes