feat(web/m7-381): admin overview Library Scan section + manual trigger

Adds getScanStatus/triggerScan helpers, createScanStatusQuery factory
(3s poll), qk.scanStatus(), and a Library Scan section on the admin
overview page with per-stage tallies and inline Run scan button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/lib/api/admin.ts

@@ -181,3 +181,57 @@ export type RefetchMissingResponse = {
 export async function refetchMissingCovers(): Promise<RefetchMissingResponse> {
   return api.post<RefetchMissingResponse>('/api/admin/covers/refetch-missing', {});
 }
+
+// Library scan -------------------------------------------------------------
+
+export type ScanStageLibrary = {
+  scanned: number;
+  added: number;
+  updated: number;
+  skipped: number;
+  errored: number;
+};
+
+export type ScanStageMbidBackfill = {
+  processed: number;
+  healed: number;
+  skipped: number;
+};
+
+export type ScanStageCoverEnrich = {
+  processed: number;
+  succeeded: number;
+  failed: number;
+};
+
+export type ScanStatus = {
+  id: string;
+  started_at: string;
+  finished_at: string | null;
+  library?: ScanStageLibrary;
+  mbid_backfill?: ScanStageMbidBackfill;
+  cover_enrich?: ScanStageCoverEnrich;
+  error_message?: string;
+  in_flight: boolean;
+};
+
+export async function getScanStatus(): Promise<ScanStatus> {
+  return api.get<ScanStatus>('/api/admin/scan/status');
+}
+
+export type TriggerScanResp = { id?: string };
+
+export async function triggerScan(): Promise<TriggerScanResp> {
+  return api.post<TriggerScanResp>('/api/admin/scan/run', {});
+}
+
+// Polls every 3s while a scan is in flight; falls back to a 30s stale-time
+// when idle so the section auto-updates as workers complete each stage.
+export function createScanStatusQuery() {
+  return createQuery({
+    queryKey: qk.scanStatus(),
+    queryFn: getScanStatus,
+    staleTime: 30_000,
+    refetchInterval: 3_000
+  });
+}