Quotas: save their usage after every change

The server hooks the engine's OnChange to save what the quotas used, so
a reboot soon after a change that counts a quota differently doesn't
bring back its usage counted the old way (it was saved every five
minutes and on stop only).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/rules.md

@@ -209,8 +209,8 @@ Statistics**.
 
 nftables counts a quota but never resets it, and forgets it when its
 table is replaced: Housecarl carries what each quota used across every
-change and restart, saves it every five minutes (and when housecarld
-stops) for after a reboot, and resets quotas when their period ends.
+change and restart, saves it every five minutes (and after each change,
+and when housecarld stops) for after a reboot, and resets quotas when their period ends.
 A per-address quota counts an address: a device that gets a new one
 starts again. With Steward's reservations IPv4 addresses stay; IPv6 is
 grouped by /64 (below).