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.

src/lib/types.ts

@@ -47,5 +47,6 @@ export interface RunState {
 	rate: number;
 	eta: number | null;
 	cancelled: boolean;
+	paused?: boolean;
 	error?: string;
 }