Supervisor for all PubSub instances used by ExESDBGater.

This supervisor is designed to be a singleton per node - it manages global PubSub instances that are shared across all ExESDB systems running on the same node.

Multiple ExESDB systems can safely attempt to start this supervisor; if it's already running, the start_link call will return {:ok, pid} with the existing supervisor's pid rather than failing.

This enables umbrella applications and multi-store setups where multiple ExESDB.System instances need to share the same PubSub infrastructure.