feat(notify): own webhook as a full notification channel

The custom webhook could be entered but never switched on since the
notification matrix replaced the old alarm channels. It is now a third
column next to e-mail and Discord for every event and the daily summary;
bundles go out as JSON (event "digest").

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

internal/api/alarms.go

@@ -140,10 +140,8 @@ func (s *Server) startAlarmTicker() {
 			if set.Events.Due.Discord {
 				channels = append(channels, "discord")
 			}
-			for _, ch := range a.AlarmChannels {
-				if ch == "webhook" {
-					channels = append(channels, "webhook")
-				}
+			if set.Events.Due.Webhook {
+				channels = append(channels, "webhook")
 			}
 			if a.AlarmEmail == "" {
 				a.AlarmEmail = a.UserEmail