New admin handler returns the library-wide cover-art coverage rollup
{total, with_art, pending, settled, pending_no_mbid} for the admin
dashboard gauge. Always 200; zeros on empty library. Same RequireAdmin
middleware as /api/admin/scan/status. Lives under a new /library/
admin sub-namespace, parallel to /scan/ (per-run state) and /covers/
(actions).
Tests gated on MINSTREL_TEST_DATABASE_URL: empty library returns all
zeros, mixed rows produce correct bucket math (verifying the
with_art + pending + settled = total invariant), non-admin returns 403.
bvandeusen
a1d295ebac