feat(engine): add query result retention policy (#123)

[shefeek-jinnah]

Summary

• Add background worker that periodically deletes expired query results (ready/failed) older than a configurable retention period
• Schedule associated Parquet files for deletion when results are cleaned up
• Configurable via result_retention_days (default: 7, set to 0 to disable)

Details

Complements the existing stale result cleanup worker (which marks stuck pending/processing results as failed) with a second phase that garbage-collects terminal results after a retention window.

• Config: RUNTIMEDB_ENGINE__RESULT_RETENTION_DAYS env var or engine.result_retention_days in config
• Worker: Runs every hour, deletes terminal results older than the cutoff and schedules their Parquet directories for deletion
• Catalog: delete_expired_results implemented for Postgres (DELETE...RETURNING) and SQLite (SELECT + DELETE)
• Lifecycle: Worker handle stored on RuntimeEngine, gracefully shut down during engine termination