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/Controllers/NotificationGroupController.php

@@ -214,6 +214,7 @@ class NotificationGroupController extends Controller
                     break;
                 case 'discord':
                 case 'slack':
+                case 'webhook':
                     $missingField = 'webhook URL';
                     break;
             }
@@ -427,6 +428,17 @@ class NotificationGroupController extends Controller
                 }
                 return ['webhook_url' => $webhookUrl];
 
+            case 'webhook':
+                $webhookUrl = trim($data['webhook_url'] ?? '');
+                if (empty($webhookUrl) || !filter_var($webhookUrl, FILTER_VALIDATE_URL)) {
+                    return null;
+                }
+                // Optional: Allow any HTTPS URL; prefer HTTPS for security
+                if (!str_starts_with($webhookUrl, 'https://') && !str_starts_with($webhookUrl, 'http://')) {
+                    return null;
+                }
+                return ['webhook_url' => $webhookUrl];
+
             default:
                 return null;
         }