feat(monitor): auto-prune expired maintenance windows

Background goroutine runs every 15 minutes, deletes maintenance windows
that expired beyond the retention period (default 7 days). Configurable
via UPTOP_MAINT_RETENTION env var (Go duration format).

Closes #72

internal/store/store.go

@@ -66,6 +66,7 @@ type Store interface {
 	AddMaintenanceWindow(mw models.MaintenanceWindow) error
 	EndMaintenanceWindow(id int) error
 	DeleteMaintenanceWindow(id int) error
+	PruneExpiredMaintenanceWindows(retention time.Duration) (int64, error)
 	IsMonitorInMaintenance(monitorID int) (bool, error)
 
 	// Preferences