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/indexer/src/worker.rs

@@ -35,6 +35,9 @@ pub async fn run_worker(state: AppState, interval_seconds: u64) {
             if let Err(err) = scheduler::check_and_schedule_reading_status_push(&scheduler_state.pool).await {
                 error!("[SCHEDULER] Reading status push error: {}", err);
             }
+            if let Err(err) = scheduler::check_and_schedule_download_detection(&scheduler_state.pool).await {
+                error!("[SCHEDULER] Download detection error: {}", err);
+            }
             tokio::time::sleep(scheduler_wait).await;
         }
     });