feat(backend): implement WebSocket authentication guard (#188)

[mijinummi]

Summary

This PR secures all WebSocket gateways with JWT authentication, ensuring only authorized users can connect and providing user context for real-time features.

Changes

• Added WsJwtAuthGuard to validate JWTs from socket handshake
• Attached authenticated user context to socket
• Secured messaging and notifications gateways
• Added tests for guard and gateway authentication

Acceptance Criteria Covered

• ✅ Unauthorized users cannot connect
• ✅ Authenticated user context available in gateways
• ✅ Guard implemented and applied
• ✅ Tests validate pause behavior

Next Steps

• Extend guard to other WebSocket namespaces
• Add monitoring for unauthorized connection attempts
• Integrate with audit logging
  Closes Implement WebSocket Authentication Guard #188