Bulwark asks for a 90-day push subscription, but Stalwart clamps
`expires` to 7 days. The background renewal ran once per page load,
so a tab or installed app left open for more than a week let the
subscription lapse and notifications stopped without a hint.
The renewal now runs at most once a day instead of once per load, and
also when the tab comes back to the foreground.
Linus Rath
b217dba461