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+# Backup --days Parameter Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Replace `--start`/`--end` date range params on the `backup` command with a single `--days N` param that backs up all orders older than N days.
+
+**Architecture:** The `backup_orders` function currently takes `start`/`end` datetime pair and filters `[start, end)`. We change it to accept a single `cutoff` datetime and filter `OrderTime < cutoff`. The CLI computes `cutoff = today_00_utc - timedelta(days=N)`.
+
+**Tech Stack:** Python, Click, pytest
+
+---
+
+### Task 1: Update `backup.py` — change function signature and filter logic
+
+**Files:**
+- Modify: `src/order_bak/backup.py`
+
+- [ ] **Step 1: Write the failing test**
+
+In `tests/test_backup.py`, add a new test that uses the `cutoff` signature:
+
+```python
+def test_backup_filters_by_cutoff(tmp_path):
+    scan_dir = tmp_path / "scan"
+    backup_dir = tmp_path / "backups"
+    scan_dir.mkdir()
+    backup_dir.mkdir()
+
+    entities = [
+        _make_entity("u1", "o1", datetime(2025, 1, 1, 10, tzinfo=timezone.utc), 1),
+        _make_entity("u2", "o2", datetime(2025, 1, 2, 8, tzinfo=timezone.utc), 1),
+        _make_entity("u3", "o3", datetime(2025, 1, 3, 8, tzinfo=timezone.utc), 1),  # at/after cutoff
+    ]
+    scan_csv = scan_dir / "scan-2025-01-03.csv"
+    write_csv(scan_csv, entities)
+
+    cutoff = datetime(2025, 1, 3, tzinfo=timezone.utc)
+    result = backup_orders(
+        scan_path=scan_csv,
+        backup_dir=backup_dir,
+        cutoff=cutoff,
+    )
+
+    assert result == 2
+    assert (backup_dir / "2025-01-01.csv").exists()
+    assert (backup_dir / "2025-01-02.csv").exists()
+    assert not (backup_dir / "2025-01-03.csv").exists()
+```
+
+- [ ] **Step 2: Run test to verify it fails**
+
+Run: `uv run pytest tests/test_backup.py::test_backup_filters_by_cutoff -v`
+Expected: FAIL — `backup_orders()` got unexpected keyword `cutoff`
+
+- [ ] **Step 3: Update `backup_orders` signature and filter**
+
+In `src/order_bak/backup.py`, replace the function:
+
+```python
+def backup_orders(
+    scan_path: Path,
+    backup_dir: Path,
+    cutoff: datetime,
+) -> int:
+    backup_dir.mkdir(parents=True, exist_ok=True)
+
+    by_date: dict[str, list[dict]] = defaultdict(list)
+
+    with open(scan_path, newline="") as f:
+        reader = csv.DictReader(f)
+        for raw_row in reader:
+            entity = parse_row(raw_row)
+            if entity["state"] != 1:
+                continue
+            order_time = entity["OrderTime"]
+            if order_time is None:
+                continue
+            if order_time >= cutoff:
+                continue
+            date_key = order_time.strftime("%Y-%m-%d")
+            by_date[date_key].append(entity)
+
+    total = 0
+    for date_key, rows in sorted(by_date.items()):
+        csv_path = backup_dir / f"{date_key}.csv"
+        with open(csv_path, "w", newline="") as f:
+            writer = csv.DictWriter(f, fieldnames=CSV_COLUMNS)
+            writer.writeheader()
+            writer.writerows(entity_to_row(r) for r in rows)
+        total += len(rows)
+        print(f"  {date_key}: {len(rows)} orders")
+
+    print(f"Backup complete: {total} orders across {len(by_date)} days")
+    return total
+```
+
+- [ ] **Step 4: Run test to verify it passes**
+
+Run: `uv run pytest tests/test_backup.py::test_backup_filters_by_cutoff -v`
+Expected: PASS
+
+- [ ] **Step 5: Update the old test to use `cutoff`**
+
+In `tests/test_backup.py`, replace `test_backup_filters_by_state_and_date`:
+
+```python
+def test_backup_filters_by_state_and_date(tmp_path):
+    scan_dir = tmp_path / "scan"
+    backup_dir = tmp_path / "backups"
+    scan_dir.mkdir()
+    backup_dir.mkdir()
+
+    entities = [
+        _make_entity("u1", "o1", datetime(2025, 1, 1, 10, tzinfo=timezone.utc), 1),
+        _make_entity("u2", "o2", datetime(2025, 1, 1, 12, tzinfo=timezone.utc), 0),  # Payed, skip
+        _make_entity("u3", "o3", datetime(2025, 1, 2, 8, tzinfo=timezone.utc), 1),
+        _make_entity("u4", "o4", datetime(2025, 1, 3, 8, tzinfo=timezone.utc), 1),  # at cutoff, skip
+    ]
+    scan_csv = scan_dir / "scan-2025-01-03.csv"
+    write_csv(scan_csv, entities)
+
+    result = backup_orders(
+        scan_path=scan_csv,
+        backup_dir=backup_dir,
+        cutoff=datetime(2025, 1, 3, tzinfo=timezone.utc),
+    )
+
+    assert result == 2
+    jan1 = backup_dir / "2025-01-01.csv"
+    jan2 = backup_dir / "2025-01-02.csv"
+    assert jan1.exists()
+    assert jan2.exists()
+    assert not (backup_dir / "2025-01-03.csv").exists()
+
+    from order_bak.csv_utils import read_csv
+    jan1_rows = list(read_csv(jan1))
+    assert len(jan1_rows) == 1
+    assert jan1_rows[0]["RowKey"] == "o1"
+```
+
+- [ ] **Step 6: Run all backup tests**
+
+Run: `uv run pytest tests/test_backup.py -v`
+Expected: All PASS
+
+- [ ] **Step 7: Commit**
+
+```bash
+git add src/order_bak/backup.py tests/test_backup.py
+git commit -m "refactor: change backup_orders to use cutoff param instead of start/end"
+```
+
+---
+
+### Task 2: Update `cli.py` — replace `--start`/`--end` with `--days`
+
+**Files:**
+- Modify: `src/order_bak/cli.py`
+
+- [ ] **Step 1: Update the backup command in `cli.py`**
+
+Replace the `backup` command function in `src/order_bak/cli.py`:
+
+```python
+@main.command()
+@click.option("--days", required=True, type=int, help="Backup orders older than N days (min 1)")
+@click.option("--scan", "scan_path", default=None, type=click.Path(), help="Path to scan CSV (default: latest in scan_dir)")
+@click.pass_context
+def backup(ctx, days, scan_path):
+    if days < 1:
+        click.echo("Error: --days must be at least 1")
+        raise SystemExit(1)
+
+    cfg = load_config(ctx.obj["config_path"])
+    from order_bak.backup import backup_orders
+
+    if scan_path:
+        scan_file = Path(scan_path)
+    else:
+        scan_files = sorted(cfg.scan_dir.glob("scan-*.csv"))
+        if not scan_files:
+            click.echo("No scan files found in scan_dir")
+            raise SystemExit(1)
+        scan_file = scan_files[-1]
+
+    from datetime import timedelta
+    today_utc = datetime.now(timezone.utc).replace(hour=0, minute=0, second=0, microsecond=0)
+    cutoff = today_utc - timedelta(days=days)
+
+    click.echo(f"Backing up orders before {cutoff.strftime('%Y-%m-%d')} UTC")
+    backup_orders(
+        scan_path=scan_file,
+        backup_dir=Path("./backups"),
+        cutoff=cutoff,
+    )
+```
+
+Add `from datetime import datetime, timedelta, timezone` to the top-level imports (replace the existing `from datetime import timezone`).
+
+- [ ] **Step 2: Run all tests**
+
+Run: `uv run pytest tests/ -v`
+Expected: All PASS (backup tests already updated in Task 1)
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add src/order_bak/cli.py
+git commit -m "feat: replace --start/--end with --days param on backup command"
+```
+
+---
+
+### Task 3: Add edge case tests
+
+**Files:**
+- Modify: `tests/test_backup.py`
+
+- [ ] **Step 1: Add edge case tests**
+
+Append to `tests/test_backup.py`:
+
+```python
+def test_backup_cutoff_at_midnight(tmp_path):
+    """Order exactly at cutoff midnight should be excluded."""
+    scan_dir = tmp_path / "scan"
+    backup_dir = tmp_path / "backups"
+    scan_dir.mkdir()
+    backup_dir.mkdir()
+
+    entities = [
+        _make_entity("u1", "o1", datetime(2025, 1, 3, 0, 0, 0, tzinfo=timezone.utc), 1),  # exactly at cutoff
+        _make_entity("u2", "o2", datetime(2025, 1, 2, 23, 59, 59, tzinfo=timezone.utc), 1),  # just before
+    ]
+    scan_csv = scan_dir / "scan.csv"
+    write_csv(scan_csv, entities)
+
+    result = backup_orders(
+        scan_path=scan_csv,
+        backup_dir=backup_dir,
+        cutoff=datetime(2025, 1, 3, tzinfo=timezone.utc),
+    )
+
+    assert result == 1
+    assert (backup_dir / "2025-01-02.csv").exists()
+    assert not (backup_dir / "2025-01-03.csv").exists()
+
+
+def test_backup_empty_result(tmp_path):
+    """No orders before cutoff returns 0 and no files."""
+    scan_dir = tmp_path / "scan"
+    backup_dir = tmp_path / "backups"
+    scan_dir.mkdir()
+    backup_dir.mkdir()
+
+    entities = [
+        _make_entity("u1", "o1", datetime(2025, 1, 5, 10, tzinfo=timezone.utc), 1),
+    ]
+    scan_csv = scan_dir / "scan.csv"
+    write_csv(scan_csv, entities)
+
+    result = backup_orders(
+        scan_path=scan_csv,
+        backup_dir=backup_dir,
+        cutoff=datetime(2025, 1, 1, tzinfo=timezone.utc),
+    )
+
+    assert result == 0
+    assert list(backup_dir.glob("*.csv")) == []
+```
+
+- [ ] **Step 2: Run all tests**
+
+Run: `uv run pytest tests/test_backup.py -v`
+Expected: All 4 tests PASS
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add tests/test_backup.py
+git commit -m "test: add edge cases for backup cutoff filtering"
+```