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/queries.ts

@@ -35,6 +35,7 @@ export const qk = {
   adminQuarantineActions: (limit?: number) =>
     ['adminQuarantineActions', { limit: limit ?? 50 }] as const,
   scanStatus:             ()              => ['scanStatus'] as const,
+  coverage:               ()              => ['coverage'] as const,
   suggestions:            (limit?: number) =>
     ['suggestions', { limit: limit ?? 12 }] as const,
   home:         ()                 => ['home'] as const,