feat: in-process nightly scheduler + pause at window end

Hook registers a 15-minute schedule that starts/resumes the backfill only
inside the maintenance window (no external cron or token). The backfill
pauses when the window closes and resumes next night. UI shows paused
state and last-run errors.

README.md

@@ -110,12 +110,12 @@ The admin module includes a full API reference with curl examples.
 
 ## Scheduling
 
-Scheduled runs are window-bound; an external cron can trigger them:
+The extension schedules itself **inside Directus** — no external cron, no token:
 
-```bash
-0 * * * * curl -fsS -X POST http://localhost:8055/image-previews/run \
-  -H 'Authorization: Bearer <admin-token>' -H 'Content-Type: application/json' -d '{}'
-```
+- every 15 minutes it checks whether it is inside the maintenance window;
+- if enabled, inside the window, and no run is active, it starts/resumes the backfill;
+- the backfill **pauses when the window closes** and resumes the next night (it is resumable);
+- once all tasks are done, it stays idle until the next day.
 
 `Run now` in the admin module ignores the window.