feat(FN-3008): notify when model settings fall back to defaults

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

packages/engine/src/__tests__/webhook-provider.test.ts

@@ -137,6 +137,7 @@ describe("WebhookNotificationProvider", () => {
     ["awaiting-user-review", "needs human review before it can proceed"],
     ["planning-awaiting-input", "is awaiting your input during planning"],
     ["gridlock", "Pipeline gridlocked"],
+    ["fallback-used", "Fusion recovered by switching from"],
     ["unknown-event", 'Event "unknown-event" for task My Task'],
   ])("message formatting for %s", async (event, expectedPart) => {
     fetchMock.mockResolvedValue({ ok: true, status: 200, statusText: "OK" });