feat: TransBorrow 归还人 to_dict 自动把数字 user id 映射为用户名（历史记录不再显示数字，lru_cache 防 N+1）

inventory-backend/app/models/transaction.py

@@ -1,6 +1,26 @@
 from app.extensions import db, beijing_time
 from datetime import datetime
 from sqlalchemy import func
+from functools import lru_cache
+
+
+@lru_cache(maxsize=512)
+def _borrow_user_name(uid):
+    """按用户 ID 取用户名（带缓存，避免列表页 N+1）"""
+    try:
+        from app.models.system import SysUser
+        u = SysUser.query.get(uid)
+        return u.username if u else None
+    except Exception:
+        return None
+
+
+def _display_borrow_operator(op):
+    """归还人展示映射：历史存的是数字 user id → 映射为用户名；新数据存姓名则原样返回"""
+    if op and str(op).strip().isdigit():
+        mapped = _borrow_user_name(int(op))
+        return mapped if mapped else op
+    return op
 
 
 class TransBorrow(db.Model):
@@ -48,7 +68,7 @@ class TransBorrow(db.Model):
             'expected_return_time': self.expected_return_time.strftime('%Y-%m-%d %H:%M') if self.expected_return_time else None,
             'is_returned': self.is_returned,
             'return_time': self.return_time.strftime('%Y-%m-%d %H:%M') if self.return_time else None,
-            'return_operator': self.return_operator,
+            'return_operator': _display_borrow_operator(self.return_operator),
             'return_signature': self.return_signature,
             'return_location': self.return_location,
             'location': self.location or '',