Move findings deletion into a worker using pub-sub system for `Ci::JobArtifacts::DestroyBatchService`

What does this MR do and why?

This MR introduces event worker using pub-sub to execute security_findings deletion for JobArtifacts::DestroyBatchService to avoid cross database modification.

• Publish event on artifact deletion
• Worker for findings deletion so that we can delete them async

Related to #346236 (closed)