feat(FN-2878): add webhook notification provider support

- 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

packages/core/src/settings-schema.ts

@@ -24,6 +24,10 @@ export const DEFAULT_GLOBAL_SETTINGS = {
   ntfyBaseUrl: undefined,
   ntfyEvents: ["in-review", "merged", "failed", "awaiting-approval", "awaiting-user-review", "planning-awaiting-input", "gridlock"],
   ntfyDashboardHost: undefined,
+  webhookEnabled: false,
+  webhookUrl: undefined,
+  webhookFormat: "generic",
+  webhookEvents: [],
   notificationProviders: [],
   defaultProjectId: undefined,
   setupComplete: undefined,