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/WebhookChannel.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace App\Notifications\Channels;
+
+use Illuminate\Notifications\Notification;
+use Illuminate\Support\Facades\Log;
+
+class WebhookChannel
+{
+    /**
+     * Send the given notification.
+     */
+    public function send(SendsWebhook $notifiable, Notification $notification): void
+    {
+        $webhookSettings = $notifiable->webhookNotificationSettings;
+
+        if (! $webhookSettings || ! $webhookSettings->isEnabled() || ! $webhookSettings->webhook_url) {
+            return;
+        }
+
+        // TODO: Implement actual webhook delivery
+        // This is a placeholder implementation
+        // You'll need to:
+        // 1. Get the webhook payload from $notification->toWebhook()
+        // 2. Create a job to send the HTTP POST request to $webhookSettings->webhook_url
+        // 3. Handle retries and errors appropriately
+
+        Log::info('Webhook notification would be sent', [
+            'url' => $webhookSettings->webhook_url,
+            'notification' => get_class($notification),
+        ]);
+    }
+}