feat(plumber): retention policy worker

[skipi]

📝 Description

• Added an expires_at column (plus migration and schema updates) to pipeline_requests so Plumber can mark pipelines slated for deletion
• Introduced Ppl.Retention.PolicyConsumer and Ppl.Retention.PolicyApplier: the consumer listens to organization policy apply events, converts the cutoff timestamp, and updates expires_at for matching pipelines while emitting retention.marked metrics

✅ Checklist

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