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

docs/CONFIGURATION.md

@@ -114,7 +114,10 @@ Location: `data/config.json`
     "data_dir": "data",
     "scheduler": {
         "enabled": true,
-        "interval_minutes": 60
+        "interval_minutes": 60,
+        "schedule_time": "03:00",
+        "schedule_days": ["mon", "tue", "wed", "thu", "fri", "sat", "sun"],
+        "auto_download_after_rescan": false
     },
     "logging": {
         "level": "INFO",
@@ -161,12 +164,17 @@ Source: [src/server/models/config.py](../src/server/models/config.py#L62-L66)
 
 ### 4.2 Scheduler Settings
 
-Controls automatic library rescanning.
+Controls automatic cron-based library rescanning (powered by APScheduler).
 
-| Field                        | Type | Default | Description                                  |
-| ---------------------------- | ---- | ------- | -------------------------------------------- |
-| `scheduler.enabled`          | bool | `true`  | Enable/disable automatic scans.              |
-| `scheduler.interval_minutes` | int  | `60`    | Minutes between automatic scans. Minimum: 1. |
+| Field                                  | Type         | Default                                       | Description                                                          |
+| -------------------------------------- | ------------ | --------------------------------------------- | -------------------------------------------------------------------- |
+| `scheduler.enabled`                    | bool         | `true`                                        | Enable/disable automatic scans.                                      |
+| `scheduler.interval_minutes`           | int          | `60`                                          | Legacy field kept for backward compatibility. Minimum: 1.            |
+| `scheduler.schedule_time`              | string       | `"03:00"`                                     | Daily run time in 24-h `HH:MM` format.                               |
+| `scheduler.schedule_days`              | list[string] | `["mon","tue","wed","thu","fri","sat","sun"]` | Days of the week to run the scan. Empty list disables the cron job.  |
+| `scheduler.auto_download_after_rescan` | bool         | `false`                                       | Automatically queue missing episodes for download after each rescan. |
+
+Valid day abbreviations: `mon`, `tue`, `wed`, `thu`, `fri`, `sat`, `sun`.
 
 Source: [src/server/models/config.py](../src/server/models/config.py#L5-L12)