Enhance security in backend server

[HendricksJudy]

Add input validation, error handling, rate limiting, and authentication to the Flask application, and update the Dockerfile to use a non-root user.

• Flask Application (sam2/demo/backend/server/app.py)

  • Add input validation and error handling in the propagate_in_video function.
  • Implement rate limiting using Flask-Limiter for API endpoints.
  • Implement authentication using Flask-HTTPAuth for API endpoints.
  • Add security headers using Flask-Talisman.
  • Protect routes with authentication and rate limiting.

• Dockerfile (sam2/backend.Dockerfile)

  • Create a non-root user and switch to it for running the application.