ci: fix environment variables setup in GitHub Actions workflow

[jdhoffa]

This PR improves the GitHub Actions workflow configuration to properly handle environment variables and secrets.

Changes:

• Fixed missing environment variables in the Docker Compose setup
• Added direct values for essential variables (ports, model names, log levels)
• Properly separated sensitive information (passwords, API keys) as secrets
• Improved variable references using GitHub's environment variables syntax
• Added documentation on proper usage of repository variables and secrets

Benefits:

• Resolves CI failures due to missing environment variables
• Ensures sensitive data is properly protected
• Provides consistent environment for system tests
• Maintains default values for non-sensitive configuration

The workflow now correctly sets up the database, API, and LLM engine variables needed for Docker Compose to run the system tests successfully.

[Copilot]

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