docs: add design spec for backup --days parameter

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/superpowers/specs/2026-04-21-backup-days-param-design.md

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+# 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/)