Add basic WebhookChannel infrastructure:
- Create SendsWebhook interface
- Create WebhookChannel with placeholder implementation (logs instead of sending)
- Update Test notification to support webhook channel
- Add WebhookChannel to HasNotificationSettings trait
- Add toWebhook() method to Test notification
This provides a working foundation that won't break test notifications.
The actual HTTP webhook delivery will be implemented in a follow-up.
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Co-Authored-By: Claude <noreply@anthropic.com>
