Merges FN-3008 to add a "fallback-used" notification system: the engine now emits events when AI model fallbacks are triggered, dispatches notifications via ntfy/webhook providers, surfaces a session banner in the dashboard, and exposes a settings toggle to enable or disable these alerts.
Fusion-Task-Id: FN-3008
- Add webhook settings fields and defaults for enablement, URL, format, and event filtering
- Implement WebhookNotificationProvider with payload formatting support for generic, Slack, and Discord endpoints
- Extend NotificationService to manage both ntfy and webhook providers with live settings sync
- Export webhook notification types/providers through engine notification entry points
