test: add edge cases for backup cutoff filtering

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

tests/test_backup.py

@@ -80,3 +80,51 @@ def test_backup_filters_by_cutoff(tmp_path):
     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()
+
+
+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")) == []