An admin sees flows across all repositories in one view #9594

From #9434 box 6.

Done looks like:

• [x] /admin/flows lists every repository's lines with state and last moved, filterable by kind and by stale.
• [x] Only a site admin may read it; a test asserts others get 404.
• [x] It reads stored state only and answers under 50 ms warm for 300 repositories.

Files: src/web/pages/, src/flows/view.ts, test/.

Reviewed pull #9620 at its head: ticked /admin/flows lists every repository's lines with state and last moved, filterable by kind and by stale.; Only a site admin may read it; a test asserts others get 404.; It reads stored state only and answers under 50 ms warm for 300 repositories..