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.

README.md

@@ -217,6 +217,34 @@ All application and email settings are now managed through the **Settings** page
 4. Copy the webhook URL
 5. Add it in the notification group settings
 
+#### 🌐 Webhook (Custom)
+
+Send JSON payloads to any HTTP endpoint (e.g., n8n, Zapier, Make, your own API):
+
+1. Go to Notification Groups → Edit → Add Channel
+2. Choose "Webhook (Custom)"
+3. Paste your endpoint URL (HTTPS recommended)
+4. Click "Test Channel" to verify
+
+Payload example sent on domain alerts:
+
+```json
+{
+  "event": "domain_expiration_alert",
+  "message": "⚠️ WARNING: Domain 'example.com' expires in 7 days (January 30, 2026)!\n\nRegistrar: Example Registrar\nPlease renew soon.",
+  "data": {
+    "domain": "example.com",
+    "domain_id": 123,
+    "days_left": 7,
+    "expiration_date": "2026-01-30",
+    "registrar": "Example Registrar"
+  },
+  "sent_at": "2025-10-17T12:34:56Z"
+}
+```
+
+Use this with n8n's "Webhook" trigger to start flows.
+
 ## 📅 Setting Up Cron Jobs
 
 The application requires a cron job to check domains periodically.