[Backend] Draw vulnerabilities over time chart in the exported PDF

What does this MR do and why?

Adds support for rendering a vulnerabilities over time chart in PDF vulnerability export reports.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

For Group:

1. Set up test data:
   • Create a group with at least one project
   • Ensure the project has vulnerabilities (you can create test vulnerabilities or run security scans)
2. Export the PDF:
   • Navigate to the Security Dashboard for the group or project
   • Click the Export button
3. Validate:
   • Download the exported PDF
   • Verify that the vulnerabilities over time is rendered correctly
   • Check that the chart displays the correct data and formatting

For Project:

1. Set up test data:
   • Create a project with vulnerabilities
2. Export the PDF:
   • Navigate to the Security Dashboard for the project
   • Click the Export button
3. Validate:
   • Download the exported PDF
   • Verify that the vulnerabilities over time is rendered correctly and that total is score is not present
   • Check that the chart displays the correct data and formatting

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #592952 (closed)