Add generic webhook notification channel

Introduces a new 'Webhook (Custom)' notification channel allowing users to send JSON payloads to any HTTP endpoint (e.g., n8n, Zapier, custom APIs). Updates the UI to support webhook configuration, adds backend validation, and implements the WebhookChannel for sending notifications. Documentation is updated with usage instructions and payload examples.

app/Services/NotificationService.php

@@ -6,6 +6,7 @@ use App\Services\Channels\EmailChannel;
 use App\Services\Channels\TelegramChannel;
 use App\Services\Channels\DiscordChannel;
 use App\Services\Channels\SlackChannel;
+use App\Services\Channels\WebhookChannel;
 
 class NotificationService
 {
@@ -18,6 +19,7 @@ class NotificationService
             'telegram' => new TelegramChannel(),
             'discord' => new DiscordChannel(),
             'slack' => new SlackChannel(),
+            'webhook' => new WebhookChannel(),
         ];
     }