fix(backend): fix sync throw isolation in event bus + add 6 tests

Promise.allSettled only catches rejected promises, not synchronous throws.
Wrap handler calls in Promise.resolve().then() to isolate sync errors.
Add 6 unit tests covering delivery, unsubscribe, error isolation,
singleton, reset, and removeAll.

backend/src/lib/event-bus.test.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { getEventBus, _resetEventBus } from './event-bus.js';
+
+describe('DomainEventBus', () => {
+  beforeEach(() => { _resetEventBus(); });
+
+  it('delivers event to subscriber', async () => {
+    const bus = getEventBus();
+    const handler = vi.fn();
+    bus.on('note.created', handler);
+    await bus.emit('note.created', { noteId: 'n1', workspaceId: 'w1', userId: 'u1', title: 'Test' });
+    expect(handler).toHaveBeenCalledOnce();
+  });
+
+  it('unsubscribe stops delivery', async () => {
+    const bus = getEventBus();
+    const handler = vi.fn();
+    const unsub = bus.on('note.created', handler);
+    unsub();
+    await bus.emit('note.created', { noteId: 'n1', workspaceId: 'w1', userId: 'u1', title: 'Test' });
+    expect(handler).not.toHaveBeenCalled();
+  });
+
+  it('failing handler does not block others', async () => {
+    const bus = getEventBus();
+    const good = vi.fn();
+    bus.on('note.created', () => { throw new Error('boom'); });
+    bus.on('note.created', good);
+    await bus.emit('note.created', { noteId: 'n1', workspaceId: 'w1', userId: 'u1', title: 'Test' });
+    expect(good).toHaveBeenCalledOnce();
+  });
+
+  it('singleton returns same instance', () => {
+    expect(getEventBus()).toBe(getEventBus());
+  });
+
+  it('_resetEventBus creates fresh instance', () => {
+    const a = getEventBus();
+    _resetEventBus();
+    expect(getEventBus()).not.toBe(a);
+  });
+
+  it('removeAll clears handlers', async () => {
+    const bus = getEventBus();
+    const handler = vi.fn();
+    bus.on('note.created', handler);
+    bus.removeAll();
+    await bus.emit('note.created', { noteId: 'n1', workspaceId: 'w1', userId: 'u1', title: 'Test' });
+    expect(handler).not.toHaveBeenCalled();
+  });
+});

backend/src/lib/event-bus.ts

@@ -61,7 +61,7 @@ class DomainEventBus {
   async emit<K extends keyof NoteLettEventMap>(event: K, payload: NoteLettEventMap[K]): Promise<void> {
     const fns = this.handlers.get(event);
     if (!fns || fns.size === 0) return;
-    await Promise.allSettled([...fns].map(fn => fn(payload)));
+    await Promise.allSettled([...fns].map(fn => Promise.resolve().then(() => fn(payload))));
   }
 
   removeAll(): void {