feat: add reading_status_push auto-refresh schedule per library

- Migration 0059: reading_status_push_mode / last / next columns on libraries
- API: update_reading_status_provider accepts push_mode and calculates next_push_at
- job_poller: handles reading_status_push pending jobs
- Indexer scheduler: check_and_schedule_reading_status_push every minute
- Backoffice: schedule select in library settings modal

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

apps/api/src/reading_status_push.rs

@@ -296,7 +296,7 @@ struct SeriesInfo {
     anilist_url: Option<String>,
 }
 
-async fn process_reading_status_push(
+pub async fn process_reading_status_push(
     pool: &PgPool,
     job_id: Uuid,
     library_id: Uuid,