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/__tests__/global-settings.test.ts

@@ -317,7 +317,16 @@ describe("GlobalSettingsStore", () => {
       // After clear, reading back gives the default value
       // (either undefined on disk with default applied, or default written directly)
       const settings = await store.getSettings();
-      expect(settings.ntfyEvents).toEqual(["in-review", "merged", "failed", "awaiting-approval", "awaiting-user-review", "planning-awaiting-input", "gridlock"]);
+      expect(settings.ntfyEvents).toEqual([
+        "in-review",
+        "merged",
+        "failed",
+        "awaiting-approval",
+        "awaiting-user-review",
+        "planning-awaiting-input",
+        "gridlock",
+        "fallback-used",
+      ]);
     });
 
     it("handles concurrent updates safely via locking", async () => {

packages/core/src/__tests__/notification-dispatcher.test.ts

@@ -153,6 +153,7 @@ describe("NotificationDispatcher", () => {
       "awaiting-user-review",
       "planning-awaiting-input",
       "gridlock",
+      "fallback-used",
     ]);
     expect(DEFAULT_GLOBAL_SETTINGS.notificationProviders).toEqual([]);
   });

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,

packages/core/src/types.ts

@@ -208,7 +208,15 @@ export interface WorkflowStep {
 
 /** Input for creating a new workflow step. */
 /** Event types that can trigger ntfy notifications */
-export type NtfyNotificationEvent = "in-review" | "merged" | "failed" | "awaiting-approval" | "awaiting-user-review" | "planning-awaiting-input" | "gridlock";
+export type NtfyNotificationEvent =
+  | "in-review"
+  | "merged"
+  | "failed"
+  | "awaiting-approval"
+  | "awaiting-user-review"
+  | "planning-awaiting-input"
+  | "gridlock"
+  | "fallback-used";
 
 /** Known notification event types. Providers may support additional custom events. */
 export const NOTIFICATION_EVENTS = [
@@ -219,6 +227,7 @@ export const NOTIFICATION_EVENTS = [
   "awaiting-user-review",
   "planning-awaiting-input",
   "gridlock",
+  "fallback-used",
 ] as const;
 
 /** Notification event type. Known events plus provider-specific custom events. */
@@ -226,7 +235,7 @@ export type NotificationEvent = (typeof NOTIFICATION_EVENTS)[number] | (string &
 
 /** Standard payload shape shared across notification providers. */
 export interface NotificationPayload {
-  taskId: string;
+  taskId?: string;
   taskTitle?: string;
   taskDescription?: string;
   event: NotificationEvent;