Backfill occurrence_id to sbom_occurrences_vulnerabilities

What does this MR do and why?

sbom_occurrences_vulnerabilities.vulnerability_occurrence_id has never been backfilled, so every existing row is NULL. That blocks reliable joins from SBOM occurrences to vulnerability findings, which reachability queries and future partitioning both depend on.

The job subclasses BackfillOccurrenceIdToVulnerabilityAssociations, the base class seven sibling BBMs already use to backfill the same column on other vulnerability association tables, so no new query logic is introduced.

This field is now being set in real-time updates via !228448 (merged) - this means we're now safe to run the backfill to fill in untouched data.

References

Resolves #602158

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Related to #602158