feat(web): remove scan-schedule admin UI (scheduler retired)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/api/queries.ts

@@ -46,7 +46,6 @@ export const qk = {
   adminPlaybackErrors:    (resolved?: boolean) =>
     ['adminPlaybackErrors', { resolved: resolved ?? false }] as const,
   scanStatus:             ()              => ['scanStatus'] as const,
-  scanSchedule:           ()              => ['scanSchedule'] as const,
   coverage:               ()              => ['coverage'] as const,
   coverProviders:         ()              => ['coverProviders'] as const,
   adminUsers:             ()              => ['adminUsers'] as const,