fix(backend): HAR file URL handling in scan processor

[gander]

Description

HAR file URL handling has issues in the scan processor that need to be fixed.

Test Location:

• File: packages/backend/tests/integration/api/scan-processor.test.ts:213
• Test: Skipped test related to HAR file URL handling

Acceptance Criteria

• Investigate artifact path construction for HAR files
• Fix harUrl serialization in scan processor
• Re-enable skipped test
• Verify test passes
• Ensure HAR files are accessible via API

Related Documentation

• Roadmap - Fix HAR file URL handling
• Skipped Tests Documentation