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>

CHANGELOG.md

@@ -96,6 +96,9 @@ All notable changes to Housecarl are documented here. The format follows
   the old entries behind. Now a total starts at the addresses' sum,
   wider groups add up their addresses, and what can't be carried starts
   again with a warning (`quota_restarts`) on the change.
+- What the quotas used is saved after every change, not only every five
+  minutes: a reboot soon after a change that counts a quota differently
+  could bring back its usage counted the old way.
 - Statistics' Quotas card linked every entry to the Rules page, not to
   its rule; each now opens its rule, named. Per-address quotas were
   missing from it (and from the `quota:rN` series): they count their