Add Comprehensive Security Testing to CI Pipeline

[JohananOppongAmoateng]

This PR integrates automated security testing into the django-polymorphic CI pipeline, providing continuous security monitoring without blocking development workflows.
The security workflow generates reports instead of failing the pipeline:

• Generates reports

• Uploads all reports as GitHub Actions artifacts

• [x]Uses continue-on-error: true to prevent blocking

• Retains reports for 30 days for historical tracking
  Closes Add Comprehensive Security and Vulnerability Checks to CI #753

[JohananOppongAmoateng]

@bckohan i am not really sure which one is the best action to take whether to fail the whole pipeline if there are security issues or generate a report so i generated reports. Please advise on the preferred route or the best one

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.33%. Comparing base (32508b8) to head (739d28e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #762   +/-   ##
=======================================
  Coverage   78.33%   78.33%           
=======================================
  Files          26       26           
  Lines        1565     1565           
  Branches      247      247           
=======================================
  Hits         1226     1226           
  Misses        263      263           
  Partials       76       76           

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