feat: add WebhookChannel placeholder implementation

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.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/Notifications/Channels/SendsWebhook.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace App\Notifications\Channels;
+
+interface SendsWebhook
+{
+    public function routeNotificationForWebhook();
+}