Post embeds already carry author-provided alt text in the appview
response (images, gallery items, and video), but bskyweb dropped it when
extracting og:image thumbnails, so link previews and crawlers never saw
an image description.
Extend extractPostMedia to return thumb+alt pairs and emit
og:image:alt / twitter:image:alt after each og:image / twitter:image
when the alt text is non-empty. Video posts use the video's alt text for
the poster thumbnail. JSON-LD callers flatten via a new imageURLs helper
so image[] strings stay byte-identical to og:image.
Related to the spirit of #8033 (alt text on link card images), but on
the bskyweb OpenGraph output side.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ki4kET2FKHBfNWR6jx6ooW
Claude
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