# Backup Days Parameter Design

## Summary

Replace the `--start` / `--end` date range parameters on the `backup` command with a single `--days N` parameter that backs up all orders older than N days. The minimum value is 1 (today's data is never backed up).

## Current Behavior

```
order-bak backup --start 2025-01-01 --end 2025-02-01
```

Backs up orders where `start <= OrderTime < end` and `state == 1`.

## New Behavior

```
order-bak backup --days 7
```

Backs up all orders where `OrderTime < cutoff` and `state == 1`, where `cutoff = today_utc - timedelta(days=N)`.

Example: `--days 7` on April 21 → cutoff is April 14 00:00 UTC → backs up all orders with OrderTime before April 14.

## Changes

### cli.py

- Remove `--start` and `--end` options
- Add `--days` option: required, int, must be >= 1
- Compute `cutoff = datetime.now(timezone.utc).replace(hour=0, minute=0, second=0, microsecond=0) - timedelta(days=days)`
- Pass `cutoff` to `backup_orders()`

### backup.py

- Change signature from `backup_orders(scan_path, backup_dir, start, end)` to `backup_orders(scan_path, backup_dir, cutoff)`
- Replace filter `if order_time < start or order_time >= end` with `if order_time >= cutoff`

### test_backup.py

- Update all existing tests to pass `cutoff` instead of `start`/`end`
- Add edge cases: cutoff exactly at midnight, N=1 (only today excluded)

## Unchanged

- `scan` command, `delete` command, config, csv_utils
- State filtering (state == 1), OrderTime null check
- Output format (per-date CSV files in backups/)