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/Traits/HasNotificationSettings.php

@@ -7,6 +7,7 @@ use App\Notifications\Channels\EmailChannel;
 use App\Notifications\Channels\PushoverChannel;
 use App\Notifications\Channels\SlackChannel;
 use App\Notifications\Channels\TelegramChannel;
+use App\Notifications\Channels\WebhookChannel;
 use Illuminate\Database\Eloquent\Model;
 
 trait HasNotificationSettings
@@ -78,6 +79,7 @@ trait HasNotificationSettings
             'telegram' => TelegramChannel::class,
             'slack' => SlackChannel::class,
             'pushover' => PushoverChannel::class,
+            'webhook' => WebhookChannel::class,
         ];
 
         if ($event === 'general') {