- Add CONFIG_RETENTION_DAYS setting (default 90) to config.py
- Create retention_service.py with cleanup_expired_snapshots (parameterized SQL via make_interval)
- APScheduler IntervalTrigger runs cleanup every 24h with 1h jitter
- Prometheus counter and histogram for observability
- CASCADE FKs handle diff/change deletion automatically
- All 4 unit tests pass
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>