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/notification/notification-service.ts

@@ -228,6 +228,15 @@ export class NotificationService {
     );
   }
 
+  async dispatch(eventType: NotificationEvent, payload: NotificationPayload): Promise<void> {
+    if (!this.notificationsEnabled) {
+      return;
+    }
+
+    const dedupTaskId = payload.taskId ?? "global";
+    this.maybeNotify(dedupTaskId, eventType, payload);
+  }
+
   private createTaskPayload(task: Task, event: NotificationEvent): NotificationPayload {
     return {
       taskId: task.id,

packages/engine/src/notification/ntfy-provider.ts

@@ -33,7 +33,8 @@ type SupportedNtfyEvent =
   | "failed"
   | "awaiting-approval"
   | "awaiting-user-review"
-  | "planning-awaiting-input";
+  | "planning-awaiting-input"
+  | "fallback-used";
 
 const SUPPORTED_EVENTS = new Set<SupportedNtfyEvent>([
   "in-review",
@@ -42,6 +43,7 @@ const SUPPORTED_EVENTS = new Set<SupportedNtfyEvent>([
   "awaiting-approval",
   "awaiting-user-review",
   "planning-awaiting-input",
+  "fallback-used",
 ]);
 
 export class NtfyNotificationProvider implements NotificationProvider {
@@ -92,8 +94,9 @@ export class NtfyNotificationProvider implements NotificationProvider {
       };
     }
 
+    const taskId = payload.taskId ?? "unknown-task";
     const taskLike = {
-      id: payload.taskId,
+      id: taskId,
       title: payload.taskTitle,
       description: payload.taskDescription ?? "",
     } as Pick<Task, "id" | "title" | "description"> as Task;
@@ -107,35 +110,40 @@ export class NtfyNotificationProvider implements NotificationProvider {
 
     const contentByEvent: Record<SupportedNtfyEvent, { title: string; message: string; priority: "default" | "high" }> = {
       "in-review": {
-        title: `Task ${payload.taskId} completed`,
+        title: `Task ${taskId} completed`,
         message: `Task "${identifier}" is ready for review`,
         priority: "default",
       },
       merged: {
-        title: `Task ${payload.taskId} merged`,
+        title: `Task ${taskId} merged`,
         message: `Task "${identifier}" has been merged to main`,
         priority: "default",
       },
       failed: {
-        title: `Task ${payload.taskId} failed`,
+        title: `Task ${taskId} failed`,
         message: `Task "${identifier}" has failed and needs attention`,
         priority: "high",
       },
       "awaiting-approval": {
-        title: `Plan needs approval for ${payload.taskId}`,
+        title: `Plan needs approval for ${taskId}`,
         message: `Task "${identifier}" needs your approval before it can proceed`,
         priority: "high",
       },
       "awaiting-user-review": {
-        title: `User review needed for ${payload.taskId}`,
+        title: `User review needed for ${taskId}`,
         message: `Task "${identifier}" needs human review before it can proceed`,
         priority: "high",
       },
       "planning-awaiting-input": {
-        title: `Planning input needed for ${payload.taskId}`,
+        title: `Planning input needed for ${taskId}`,
         message: `Task "${identifier}" is awaiting your input during planning`,
         priority: "high",
       },
+      "fallback-used": {
+        title: `Fallback model used${payload.taskId ? ` for ${payload.taskId}` : ""}`,
+        message: `Fusion switched from ${String(payload.metadata?.primaryModel ?? "primary model")} to ${String(payload.metadata?.fallbackModel ?? "fallback model")} after a retryable failure (${String(payload.metadata?.triggerPoint ?? "unknown trigger")}).`,
+        priority: "high",
+      },
     };
 
     const content = contentByEvent[event as SupportedNtfyEvent];

packages/engine/src/notification/webhook-provider.ts

@@ -133,6 +133,8 @@ export class WebhookNotificationProvider implements NotificationProvider {
         return `Task "${identifier}" is awaiting your input during planning`;
       case "gridlock":
         return "Pipeline gridlocked";
+      case "fallback-used":
+        return `Fusion recovered by switching from ${String(payload.metadata?.primaryModel ?? "primary model")} to ${String(payload.metadata?.fallbackModel ?? "fallback model")} (${String(payload.metadata?.triggerPoint ?? "unknown trigger")})`;
       default:
         return `Event "${event}" for task ${identifier}`;
     }
@@ -145,7 +147,7 @@ export class WebhookNotificationProvider implements NotificationProvider {
 
     const description = payload.taskDescription ?? "";
     const snippet = description.length > 200 ? `${description.slice(0, 200)}...` : description;
-    return `${payload.taskId}: ${snippet}`;
+    return `${payload.taskId ?? "unknown-task"}: ${snippet}`;
   }
 
   private formatPayload(payload: NotificationPayload, message: string): Record<string, unknown> {