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/operations.md

@@ -164,9 +164,9 @@ start again. A disabled rule keeps its count for when it is enabled.
 resets; `hc rule reset-quota ID` starts one again (`--address` one device's,
 `--give 5gbytes` a top-up until the next reset). housecarld resets
 quotas at midnight when their period ends (late, if it was stopped then),
-and saves their usage every five minutes and when it stops; after a
-reboot it puts it back. A reboot without a clean stop loses at most the
-last five minutes' count.
+and saves their usage every five minutes, after every change and when
+it stops; after a reboot it puts it back. A reboot without a clean stop
+loses at most the last five minutes' count.
 
 ## The log