refactor: change backup_orders to use cutoff param instead of start/end

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

src/order_bak/backup.py

@@ -9,8 +9,7 @@ from order_bak.csv_utils import CSV_COLUMNS, entity_to_row, parse_row
 def backup_orders(
     scan_path: Path,
     backup_dir: Path,
-    start: datetime,
-    end: datetime,
+    cutoff: datetime,
 ) -> int:
     backup_dir.mkdir(parents=True, exist_ok=True)
 
@@ -25,7 +24,7 @@ def backup_orders(
             order_time = entity["OrderTime"]
             if order_time is None:
                 continue
-            if order_time < start or order_time >= end:
+            if order_time >= cutoff:
                 continue
             date_key = order_time.strftime("%Y-%m-%d")
             by_date[date_key].append(entity)