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>

internal/server/server_test.go

@@ -792,6 +792,21 @@ func TestQuotaResets(t *testing.T) {
 	}
 }
 
+// TestQuotaUsageSavedOnChange: a change saves what the quotas used at
+// once (a restart must not bring back usage counted the old way).
+func TestQuotaUsageSavedOnChange(t *testing.T) {
+	e := newEnv(t)
+	e.call(e.token, "POST", "/zones", model.Zone{Name: "lan", Interfaces: []string{"eth1"}}, nil)
+	e.fw.mu.Lock()
+	e.fw.quotas = nft.QuotaUsage{Quotas: map[string]int64{"quota_r7": 4242}}
+	e.fw.mu.Unlock()
+	e.call(e.token, "POST", "/zones", model.Zone{Name: "dmz", Interfaces: []string{"eth2"}}, nil)
+	v, err := e.eng.Store().GetMeta(context.Background(), QuotaUsageKey)
+	if err != nil || !strings.Contains(v, `"quota_r7":4242`) {
+		t.Errorf("saved usage %q %v", v, err)
+	}
+}
+
 // TestAutoConfirm: the client that made a change confirms it by itself:
 // with its token, from the same address, over a new connection. Nobody
 // else can, an older token can't, and with manual_confirm there is none.