[codex] Add thumbnail-first video support

[acodebeard]

Summary

• Add a media rendering helper that branches image and video APOD entries.
• Render video entries as local poster-first media with a user-activated iframe stored in data-src until playback is requested.
• Keep image entries on the existing lightbox path, while blocking lightbox markup for videos.
• Add gallery video badges, accessible view-toggle state, and video button state.
• Add CI coverage for video rendering and asset checks that reject DOM HTML-writing APIs.

Notes

• Video embed URLs are allowlisted before rendering iframe markup.
• Raw NASA API video entries still need importer normalization for local poster assets before archive ingestion.

Validation

• php scripts/check-video-support.php
• scripts/php-lint.sh
• scripts/check-source-only.sh
• scripts/check-file-modes.sh
• scripts/check-assets.sh
• git diff --check
• Local smoke tests for /apod/gallery and /apod/image/the-veins-of-heaven returned HTTP 200.