feat(09-01): create outboxTrigger.ts zero-dependency EventEmitter signal module

- Module-level singleton EventEmitter, single subscriber, no setMaxListeners
- Export signalOutboxDrain(): void — fire-and-forget drain signal (D-04)
- Export onOutboxDrain(handler): () => void — register/unsubscribe listener
- Only imports node:events; zero internal dependencies (no circular import risk)

apps/api/src/lib/outboxTrigger.ts

@@ -0,0 +1,39 @@
+/**
+ * In-process outbox drain signal (D-01 / D-03 / D-04).
+ *
+ * Module-level singleton EventEmitter — one emitter shared across all callers
+ * in this Node.js process. Carries a zero-payload 'drain' event: signal is
+ * fire-and-forget; no data crosses this boundary (D-04).
+ *
+ * Single-subscriber design:
+ *   - signalOutboxDrain() is called only after a successful enqueue DB commit (D-01/D-03).
+ *   - The sole listener is registered by initOutboxTrigger() in outboxWorker.ts.
+ *   - With exactly one subscriber the default EventEmitter limit of 10 is correct.
+ *   - Do NOT call setMaxListeners — unlike listEmitter.ts (T-04-04, 200 SSE fan-out),
+ *     this emitter never fans out to multiple listeners.
+ *
+ * Exports: signalOutboxDrain, onOutboxDrain
+ */
+
+import { EventEmitter } from 'node:events';
+
+// Module-level singleton — one emitter shared across all route handlers
+// in this Node.js process.
+const emitter = new EventEmitter();
+
+/**
+ * Fire a drain signal after a successful outbox enqueue commit.
+ * Synchronous and fire-and-forget — never awaited (D-04).
+ */
+export function signalOutboxDrain(): void {
+  emitter.emit('drain');
+}
+
+/**
+ * Register a handler to be invoked on each drain signal.
+ * Returns an unsubscribe function; call it to stop delivery to this handler.
+ */
+export function onOutboxDrain(handler: () => void): () => void {
+  emitter.on('drain', handler);
+  return () => emitter.off('drain', handler);
+}