feat: cron-based scheduler with auto-download after rescan

- Replace asyncio sleep loop with APScheduler AsyncIOScheduler + CronTrigger
- Add schedule_time (HH:MM), schedule_days (days of week), auto_download_after_rescan fields to SchedulerConfig
- Add _auto_download_missing() to queue missing episodes after rescan
- Reload config live via reload_config(SchedulerConfig) without restart
- Update GET/POST /api/scheduler/config to return {success, config, status} envelope
- Add day-of-week pill toggles to Settings -> Scheduler section in UI
- Update JS loadSchedulerConfig / saveSchedulerConfig for new API shape
- Add 29 unit tests for SchedulerConfig model, 18 unit tests for SchedulerService
- Rewrite 23 endpoint tests and 36 integration tests for APScheduler behaviour
- Coverage: 96% api/scheduler, 95% scheduler_service, 90% total (>= 80% threshold)
- Update docs: API.md, CONFIGURATION.md, features.md, CHANGELOG.md

requirements.txt

@@ -17,4 +17,5 @@ sqlalchemy>=2.0.35
 aiosqlite>=0.19.0
 aiohttp>=3.9.0
 lxml>=5.0.0
-pillow>=10.0.0
+pillow>=10.0.0
+APScheduler>=3.10.4