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>

frontend/src/lib/types.ts

@@ -49,6 +49,7 @@ export type SharingMode = 'private' | 'organizations' | 'open';
 export interface NotificationChannelRouting {
   email: boolean;
   discord: boolean;
+  webhook: boolean;
 }
 
 export interface NotificationEvents {
@@ -63,6 +64,7 @@ export interface DailySummarySettings {
   time: string;
   email: boolean;
   discord: boolean;
+  webhook: boolean;
 }
 
 export interface NotificationSettings {