Quotas: carry what was used when a rule counts differently

model.QuotaCarry says what becomes of a quota's use when its rule
changes how it counts; the firewall's apply follows it, comparing with
the rules in force: per address to a total starts at the addresses' sum,
wider groups add up their addresses, and what can't be carried (a total
to per address, per source to per destination, finer groups) starts
again, with the engine warning quota_restarts on that change. Before,
per source to per destination kept the sources' use as destinations',
and regrouping left old entries behind.

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

CHANGELOG.md

@@ -87,6 +87,12 @@ All notable changes to Housecarl are documented here. The format follows
 
 ### Fixed
 
+- Changing how a quota counts lost or muddled what it used: per address
+  to a total started at nothing, per source to per destination kept the
+  sources' use as if it were the destinations', and other groups left
+  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.
 - `hc stats top devices` listed each device twice (once for what it
   received, once for what it sent); it is one row with both again.
 - `hc rule add --before/--after/--first` made two changes (create at the