feat: extend retention policies

[skipi]

📝 Description

• Plumber now emits deletion events - when pipelines expire and are deleted, plumber publishes PipelineDeleted and WorkflowDeleted events to RabbitMQ. Workflow deletion is only emitted once all pipelines in a workflow are gone.
• Artifacthub gains two new workers - a pipelinedeletion and workflowdeletion worker that consume those events and clean up the associated artifacts in the bucket.
• Plumber's retention internals were refactored - the old policy consumer/record deleter was split into two explicit workers (policy marker and deleter) with cleaner separation of concerns, replacing the previous policy_applier/record_deleter structure.

✅ Checklist

• I have tested this change
• This change requires documentation update