fix(catchup): guard on real in-flight backup check; add scheduler tests

internal/alert/engine.go

@@ -240,6 +240,9 @@ func (e *Engine) tick(ctx context.Context, now time.Time) {
 			e.raiseAndNotify(ctx, h.ID, KindStaleSchedule, "", "warning",
 				fmt.Sprintf("No backup in %s (threshold %s)",
 					roundDur(now.Sub(*h.LastBackupAt)), staleBackupThreshold), now)
+			// Resolution is handled in handleJobFinished on a successful
+			// backup (and ResolveOnModeChange on toggle) — the tick only
+			// raises, it does not auto-resolve.
 			continue
 		}
 		// Always-on hosts: existing agent_offline re-evaluation.