Media Library doesn't expose pagination — users with >50 files can't access them

[sheadscott]

Summary

The media library hard-stops at 50 items with no way to load more. Users who have uploaded
more than 50 files silently lose access to the rest.

The backend already supports cursor-based pagination. GET /_emdash/api/media returns a
nextCursor in the response whenever more items exist. The infrastructure is there and working
— the admin UI simply never uses it.

What's missing in packages/admin/:

• MediaLibrary.tsx receives items as a plain prop — the caller fetches everything and passes
  it down, with no cursor state
• router.tsx (the Media page) makes a single flat query with no useInfiniteQuery or cursor
  tracking
• For external providers, fetchProviderMedia() is called with a hardcoded limit: 50 and the
  returned nextCursor is discarded

Proposed fix (UI-only, no API changes needed):

1. Switch the Media page in router.tsx from a plain query to useInfiniteQuery, passing
   nextCursor as the pageParam
2. Add a onLoadMore / hasMore prop to MediaLibrary
3. Render a "Load more" button (or implement intersection-observer infinite scroll) when
   hasMore is true
4. Apply the same pattern to the provider media query inside MediaLibrary itself

Happy to implement this if maintainers want it. The backend needs no changes.

[ascorbic]

Fixed in #996. Thanks!