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/server.go

@@ -38,6 +38,7 @@ type Server struct {
 	// Replace the delivery of bundled notifications in tests (notify.go)
 	notifyEmailHook   func(to, subject, text, html string) error
 	notifyDiscordHook func(webhookURL, heading string, items []notifyItem) error
+	notifyWebhookHook func(webhookURL, heading string, items []notifyItem) error
 }
 
 func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS fs.FS) *Server {