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.
Refactored error log model and views to use a unified 'notes' field instead of 'resolution_notes'. Added a modal dialog for entering resolution notes when marking errors as resolved in admin views. Improved stack trace handling in ErrorHandler by storing as JSON and formatting for display. Expanded NotificationService to support multi-channel notifications (email, Telegram, Discord, Slack), group notifications, and improved domain expiration alerts.
