Refactor JWT Secret Handling for Improved Security and Consistency

[Leon-JavaScript]

This PR refactors the JWT secret management in auth.ts to enhance security and maintain consistent usage. The changes include:

• Using a single JWT_SECRET variable, with a default if not set.
• Adding a warning if the secret is missing or set to the insecure default value.
• Removing the redundant ACTUAL_JWT_SECRET variable.
• Updating token generation and verification to use the unified JWT_SECRET.
• Improving clarity of security warnings for development and production environments.

These updates help prevent accidental usage of insecure secrets and streamline authentication logic.