Add finding_id column to vulnerability backup tables

What does this MR do and why?

Add vulnerability_occurrence_id mapping to 6 backup models to support the transition from vulnerability-centric to finding-centric models as part of the Vulnerabilities Across Contexts feature.

Affected backup tables

Backup Table	Backup Model	Source Model
backup_vulnerability_state_transitions	Backups::VulnerabilityStateTransition	Vulnerabilities::StateTransition
backup_vulnerability_severity_overrides	Backups::VulnerabilitySeverityOverride	Vulnerabilities::SeverityOverride
backup_vulnerability_issue_links	Backups::VulnerabilityIssueLink	Vulnerabilities::IssueLink
backup_vulnerability_merge_request_links	Backups::VulnerabilityMergeRequestLink	Vulnerabilities::MergeRequestLink
backup_vulnerability_external_issue_links	Backups::VulnerabilityExternalIssueLink	Vulnerabilities::ExternalIssueLink
backup_vulnerability_reads	Backups::VulnerabilityRead	Vulnerabilities::Read

Changes

1. Add database migration to add finding_id bigint column to all 6 backup tables
2. Update each backup model's column_mapping to include vulnerability_occurrence_id: :finding_id
3. Update corresponding specs

This follows the existing pattern used by other finding-related backup tables like backup_finding_flags.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to

Closes #589560