Challenge 1: Find and Fix a Listener Leak — Possible Solution
====================================================================

BEFORE (leaks a listener — and everything it closes over — on every request):

app.get("/dashboard", (req, res) => {
  bus.on("order.placed", (payload) => {
    res.write(JSON.stringify(payload));
  });
  // res is never released from the bus's listener list — after 10,000
  // requests, 10,000 dead `res` objects (and their sockets) are still
  // referenced and cannot be garbage collected.
});

AFTER (unsubscribes once the response finishes):

app.get("/dashboard", (req, res) => {
  const onOrderPlaced = (payload: { orderId: string; total: number }) => {
    res.write(JSON.stringify(payload));
  };

  bus.on("order.placed", onOrderPlaced);

  res.on("finish", () => {
    bus.off("order.placed", onOrderPlaced);
  });

  // Also handle the client disconnecting early, which doesn't always fire "finish":
  res.on("close", () => {
    bus.off("order.placed", onOrderPlaced);
  });
});

WHY THIS WORKS
--------------
- The listener is stored in a named variable (onOrderPlaced) instead of an
  inline arrow function, because bus.off() needs a reference to the exact
  same function that was passed to bus.on() — an inline arrow function
  passed separately to off() would never match and the listener would
  never actually be removed.
- Listening for both "finish" (normal completion) and "close" (client
  disconnected early) ensures the listener is cleaned up in either case —
  relying on "finish" alone would leak a listener for every request whose
  client disconnects before the response completes.
- After this fix, each request's listener has a lifetime exactly matching
  that request's lifetime — nothing outlives the request that created it.