Add daily summary aggregation that tracks apartment inventory changes:
- Upsert summaries by date key for idempotent operation
- Track new_units, rented_units, and stale_units arrays with counts
- Calculate net_change (new - rented) and total_available_today
- Fetch previous day summary for total_available_yesterday
- Compute turnover_rate as percentage of rented vs yesterday available
- Handle edge cases: zero division, missing previous day, month boundaries
- Full test coverage with 22 tests across all functional areas
