fix(09): WR-01 defer outbox drain emit to a microtask so a throwing listener cannot corrupt the enqueue route response

apps/api/src/lib/outboxTrigger.ts

@@ -23,10 +23,18 @@ const emitter = new EventEmitter();
 
 /**
  * Fire a drain signal after a successful outbox enqueue commit.
- * Synchronous and fire-and-forget — never awaited (D-04).
+ * Fire-and-forget — never awaited (D-04).
+ *
+ * WR-01: the emit is deferred to a microtask so the registered listener
+ * (scheduleOutboxDrain) never runs on the enqueue caller's stack. EventEmitter.emit
+ * dispatches listeners synchronously; without this defer, any synchronous throw in
+ * the listener chain would propagate back into the route handler's try/catch and
+ * surface a misleading 503 for an enqueue that actually committed. Deferring keeps
+ * the documented decoupling honest: a misbehaving listener can never corrupt the
+ * route response.
  */
 export function signalOutboxDrain(): void {
-  emitter.emit('drain');
+  queueMicrotask(() => emitter.emit('drain'));
 }
 
 /**