refactor(eventbus): replace EventBus struct with EventBus interface

[ckeyer]

• Remove unused EventBus struct and NewEventBus function (dead code)
• Replace PublisherSubscriber interface with EventBus interface for better semantics
• Update SetupRouter to accept EventBus interface instead of concrete Publisher type
• Enable SessionService to use the same NATS connection for both publish and subscribe

This change improves code architecture by:

1. Removing dead code (EventBus struct was never used)
2. Using idiomatic Go interface composition
3. Ensuring single NATS connection throughout the application
4. Better semantic naming (EventBus > PublisherSubscriber)