Add WebSocket event gateway for real-time cross-service notifications

[amaydixit11]

Description

The Hub currently serves as an API gateway/broker but has no real-time event distribution capability. Services need to push real-time notifications (new messages, application updates, new listings) across the ecosystem but must rely on individual service implementations.

Proposed Feature

1. WebSocket event gateway: Centralized WebSocket server that all downstream services can publish events to
2. Event bus: Publish/subscribe pattern for inter-service communication
3. Client subscription channels: Clients can subscribe to specific event types (marketplace, jobs, events, etc.)
4. Event persistence: Store events for offline users with replay on reconnection

Technical Approach

• Integrate @nestjs/websockets with the existing NestJS structure
• Define a standard event schema (type, source, payload, timestamp)
• Each downstream service publishes to Hub's event bus via HTTP or internal gRPC
• Hub routes events to subscribed WebSocket clients based on user interests and roles