A per-item Open Graph card, so a shared listing link previews as the app

[justinmerrell]

musher.dev/catalog/{slug} currently serves the generic site card
(/og-image.png) as its og:image and twitter:image. Every listing page —
and every ad, Slack paste and shared link pointing at one — previews as
"Musher" rather than as the app.

The listing's own icon is available and is deliberately not used for this: a
square logo in a summary_large_image card renders as a small mark on a large
empty field, which looks broken rather than sparse.

What is wanted is a 1200×630 card per item, carrying the app's mark, its name,
and the one-line summary. Two ways to get there:

1. Authored, as media/og.png beside media/icon.png in this repo — a new
   spec.ogImage media path, resolved by the importer the same way icon and
   screenshots already are. Predictable, versioned, and needs no runtime.
2. Generated at request time from the icon plus the summary, on the platform
   side. No per-item work, but it is a rendering service the platform does not
   have today.

(1) is the smaller change and fits how the rest of the item's media already
works. Sized 1200×630, under ~300 KB, with the mark large enough to read at the
~500 px width most timelines actually render.