feat(web/m7-coverage-gauge): coverage API client + query

Adds CoverageRollup type, getCoverageRollup() against
/api/admin/library/coverage, and createCoverageQuery() with the same
3s/30s pacing used by scan-status. New qk.coverage() key follows the
existing zero-arg tuple pattern. Vitest covers the GET path and the
empty-library zero-state plus the with_art + pending + settled = total
invariant.

web/src/lib/api/admin.ts

@@ -236,3 +236,29 @@ export function createScanStatusQuery() {
     refetchInterval: 3_000
   });
 }
+
+// Library coverage --------------------------------------------------------
+
+export type CoverageRollup = {
+  total: number;
+  with_art: number;
+  pending: number;
+  settled: number;
+  pending_no_mbid: number;
+};
+
+export async function getCoverageRollup(): Promise<CoverageRollup> {
+  return api.get<CoverageRollup>('/api/admin/library/coverage');
+}
+
+// Same pacing as scan-status — 3s while polling, 30s stale-time. The
+// query is cheap server-side (one aggregate per call) and we want
+// the gauge to tick during a scan.
+export function createCoverageQuery() {
+  return createQuery({
+    queryKey: qk.coverage(),
+    queryFn: getCoverageRollup,
+    staleTime: 30_000,
+    refetchInterval: 3_000
+  });
+}