[pallaoro]

A share link pasted into Slack, WhatsApp, iMessage, LinkedIn or X now unfurls with a still from the video instead of a bare title. The share page's player also shows that still before it plays.

• Every export asks the edit service for a poster: the output's middle frame, cut from the finished MP4, so it shows what viewers will see (crop, text and fades included). It is kept beside the render (renders/<name>.jpg) and served at /s/<token>/poster.jpg, with the same pin rule as the video. A stale v gets a 404, and turning the link off ends it.
• The share page carries og:image (absolute, as unfurlers require), og:image:type, og:image:alt and twitter:card=summary_large_image, but only when the export has a poster. Exports made before this, or on a service without posters, keep today's page.
• The poster is best-effort. A failed copy never fails an export.
• Deleting a project removes each export's poster along with its render.

Needs the edit-service half deployed before posters appear. Until then the field is ignored and nothing changes, so the merge order doesn't matter.

Checked: tsc, vitest (89 tests, including new ones for the meta tags and the poster key) and vite build. Locally under wrangler dev against a mock edit service, three cases: an export with a poster (the share page has the tags, the poster is served byte-identical, a stale v gets 404, HEAD gets 200, and deleting the project removes both objects); an export without a poster (no tags, poster route 404, video 200); and a poster URL that fails to download (export still completes, no tags).