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>

infra/migrations/0061_add_download_detection_schedule.sql

@@ -0,0 +1,3 @@
+ALTER TABLE libraries ADD COLUMN download_detection_mode VARCHAR NOT NULL DEFAULT 'manual';
+ALTER TABLE libraries ADD COLUMN next_download_detection_at TIMESTAMPTZ;
+ALTER TABLE libraries ADD COLUMN last_download_detection_at TIMESTAMPTZ;