fix(reporting): use timezone-aware utc datetime in exported reports

[NiravaM]

Description

The generated_at timestamp in exported reports was being populated using a naive local datetime (datetime.now()). This was inconsistent with the timezone-aware UTC discovered_at timestamps used elsewhere in the codebase, leading to off-by-timezone confusion in exported reports.

This change updates the generated_at field generation in the report payload to use a timezone-aware UTC datetime (datetime.now(timezone.utc)).

Related Issues

fixes #1829

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

Please remove the unrelated root package-lock.json; this backend timestamp fix should not add an empty frontend lockfile. Also add a focused report-payload test asserting generated_at is UTC-aware/UTC-derived, then rerun checks.