feat(events): add internal event bus with typed events, listeners, an…

[dominiccreates]

this pr closes #43 Implemented a lightweight internal event bus using NestJS EventEmitter2 to decouple module communication.

• Scaffolded src/events/:
  • events.enum.ts – enum of event names.
  • event-payloads.ts – typed payload interfaces.
  • event.service.ts – thin wrapper for emitting events.
• Integrated EventEmitterModule (global) into AppModule.
• Added placeholder modules (scoring, achievements, rewards, notifications) that subscribe to relevant events asynchronously.
• Updated package.json with @nestjs/event-emitter dependency.
• Added comprehensive unit tests (events.service.spec.ts) validating emission and listener handling for each event type.

Benefits

• Modules no longer call each other directly → easier maintenance and extension.
• New side‑effects can be added without touching existing business logic.
• Asynchronous listeners keep request cycles fast.

Testing

Run npm test – all new tests pass, confirming proper event flow.