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/queries.ts

@@ -34,6 +34,7 @@ export const qk = {
   adminQuarantine:        ()              => ['adminQuarantine'] as const,
   adminQuarantineActions: (limit?: number) =>
     ['adminQuarantineActions', { limit: limit ?? 50 }] as const,
+  scanStatus:             ()              => ['scanStatus'] as const,
   suggestions:            (limit?: number) =>
     ['suggestions', { limit: limit ?? 12 }] as const,
   home:         ()                 => ['home'] as const,