Add Comprehensive Security and Vulnerability Checks to CI

[JohananOppongAmoateng]

Description:
django-polymorphic currently lacks automated security checks in its CI pipeline. To ensure the library remains secure and robust, we should integrate full CI-based security testing.

Proposed Solution:

• Static code analysis: Use tools like Bandit to scan for common Python vulnerabilities.
• Dependency scanning: Automatically check for vulnerable dependencies using tools such as Dependabot, Safety, or pip-audit.
• CI integration: All checks should run automatically on pull requests and main branch builds, failing the pipeline if issues are detected.