Resolves event names to subscriber modules.

Reads subscribers from Outbox.Config.subscribers/0 on every lookup. No GenServer, no ETS cache — the dispatcher polls every 5 seconds, so the lookup happens at most a few times per second per node; caching would not be a meaningful optimization at this volume.

Trivial-by-design also means tests can override the subscriber list with Application.put_env/3 without restarting the application.