[Backend] Draw vulnerabilities over time chart in the exported PDF
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## Summary
Implement backend logic for exporting vulnerabilities over time.
## Required Backend Changes (4 main areas)
* JSON Schema Update - Add total_risk_score to validation schema
* New PDF Renderer Class - Create Gitlab::PDF::Security::VulnerabilitiesOverTime
* PDF Export Service Update - Integrate vulnerabilities over time rendering
* API Documentation - Update parameter documentation
task