[Composition Analysis] Backfill sbom_occurrences_vulnerabilities.vulnerability_occurrence_id (BBM)

Part of &21396 (Complete at Scale Vulnerability Management).

Problem

sbom_occurrences_vulnerabilities.vulnerability_occurrence_id has no backfill background migration (VAC consistency check VO-ID-FILL). Existing rows are NULL, which blocks reliable context joins for reachability and future partitioning.

Scope

Add a batched background migration (BBM) to backfill vulnerability_occurrence_id from vulnerabilities.finding_id. Coordinate with the VAC consistency-check work in &20929.

Notes

• This is a prerequisite for making reachability queries tracked-context aware (companion SBOM query-layer issue).