Create Events Module for Internal Domain Event Bus

[Mkalbani]

Description

Build an events module that provides a lightweight internal event bus for decoupled communication between modules. Rather than sessions directly calling scoring, rewards, achievements, and notifications, they emit events that each module listens to independently. This keeps modules loosely coupled and makes it easy to add new side effects without modifying the originating module.

Acceptance Criteria

• An events module is scaffolded at src/events/
• NestJS EventEmitter2 is used as the underlying bus
• Defined event types include: session.completed, score.updated, achievement.unlocked, reward.granted
• Events carry a typed payload — no raw any objects
• Scoring, achievements, rewards, and notifications modules subscribe to relevant events
• Event listeners run asynchronously and do not block the emitting request
• Unit tests cover event emission and listener invocation for each event type