feat: add workflow_runs table retention/cleanup

[SokratisVidros]

Description

There is no mechanism to purge old terminal workflow runs (completed/failed/cancelled) from the workflow_runs table. In a long-running deployment, this table grows unbounded.

pg-boss has its own archive/maintenance for jobs, but workflow_runs is an independent table managed by the engine.

Suggested approach

Add a configurable retention policy, e.g.:

• A retentionDays option in WorkflowEngineOptions
• A periodic cleanup that deletes terminal runs older than the retention period
• Optionally, an engine.purgeRuns() method for manual cleanup

Location

src/engine.ts, src/db/queries.ts