Add authentication middleware to protect API endpoints

[AbdulSnk]

Overview

The StreamForge API endpoints need to be protected so that only authorized services and users can access contribution and scoring data.

What needs to be done

• Implement a JWT-based authentication middleware in NestJS
• Create an auth module under src/
• Protect all routes except public health check endpoints
• Implement the following:
  • POST /auth/login — accepts credentials and returns a signed JWT token
  • Auth guard that validates JWT on every protected request
  • Proper error responses for unauthorized requests

Acceptance Criteria

• src/auth module exists with guard, service, and controller
• POST /auth/login returns a valid JWT on correct credentials
• Protected routes return 401 when no token is provided
• Protected routes return 403 when token is invalid or expired
• At least one test covers authorized and unauthorized access

Notes

Use the @nestjs/jwt and @nestjs/passport packages. Follow NestJS guard conventions. Do not hardcode secrets — use environment variables.