fix(scheduler): implement scheduler daemon and resolve database/API issues
- Add SchedulerService with background cron monitoring loop
- Fix manual schedule execution to use scheduler service instead of creating pending records
- Fix database type error: serialize items_processed dict to JSON string before saving
- Fix Jellyfin API calls: ensure settings loaded before making requests
- Fix session rollback errors in exception handling
- Update database session usage to async generator pattern throughout scheduler
- Add croniter dependency for cron expression parsing
- Integrate scheduler with existing job system for poster processing
- Start/stop scheduler service with API lifecycle in main.py
- Update job_service to handle "scheduled_poster_processing" job type
Fixes: Schedules now execute automatically and manual execution works properly
Resolves: Schedule executions progress from pending → processing → completed instead of staying stuck
Resolves: Database serialization errors and session rollback issues