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/domain/models.go

@@ -157,13 +157,18 @@ type NotificationEvents struct {
 type ChannelChoice struct {
 	Email   bool `json:"email"`
 	Discord bool `json:"discord"`
+	Webhook bool `json:"webhook"` // own webhook (customWebhookUrl), JSON payload
 }
 
+// Any reports whether at least one channel is chosen.
+func (c ChannelChoice) Any() bool { return c.Email || c.Discord || c.Webhook }
+
 type DailySummary struct {
 	Enabled bool   `json:"enabled"`
 	Time    string `json:"time"` // HH:MM, server time zone
 	Email   bool   `json:"email"`
 	Discord bool   `json:"discord"`
+	Webhook bool   `json:"webhook"`
 }
 
 // DefaultNotificationSettings for users who never saved any; task alarms follow the old alarm channels.
@@ -172,6 +177,7 @@ func DefaultNotificationSettings(alarmChannels []string) NotificationSettings {
 	for _, c := range alarmChannels {
 		due.Email = due.Email || c == "email"
 		due.Discord = due.Discord || c == "discord"
+		due.Webhook = due.Webhook || c == "webhook"
 	}
 	return NotificationSettings{
 		Events: NotificationEvents{