feat: add per-library download detection auto-schedule

Adds a configurable schedule (manual/hourly/daily/weekly) for the
download detection job in the library settings modal. The indexer
scheduler triggers the job automatically, and the API job poller
processes it — consistent with the reading_status_push pattern.

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

apps/backoffice/app/(app)/libraries/page.tsx

@@ -148,6 +148,7 @@ export default async function LibrariesPage() {
                       metadataRefreshMode={lib.metadata_refresh_mode}
                       readingStatusProvider={lib.reading_status_provider}
                       readingStatusPushMode={lib.reading_status_push_mode}
+                      downloadDetectionMode={lib.download_detection_mode ?? "manual"}
                     />
                     <form>
                       <input type="hidden" name="id" value={lib.id} />