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/core/src/settings-schema.ts

@@ -23,7 +23,16 @@ export const DEFAULT_GLOBAL_SETTINGS = {
   ntfyEnabled: false,
   ntfyTopic: undefined,
   ntfyBaseUrl: undefined,
-  ntfyEvents: ["in-review", "merged", "failed", "awaiting-approval", "awaiting-user-review", "planning-awaiting-input", "gridlock"],
+  ntfyEvents: [
+    "in-review",
+    "merged",
+    "failed",
+    "awaiting-approval",
+    "awaiting-user-review",
+    "planning-awaiting-input",
+    "gridlock",
+    "fallback-used",
+  ],
   ntfyDashboardHost: undefined,
   webhookEnabled: false,
   webhookUrl: undefined,