feat(borrow): 借库申请后端必填校验——申请原因、预计归还日期(长期借用除外)

- submit_approval 校验：申请原因非空；未勾长期借用时必须有 expected_return_time，否则 ValueError 拦截

inventory-backend/app/services/borrow_service.py

@@ -88,6 +88,14 @@ class BorrowApprovalService:
         if not items:
             raise ValueError("借库物品列表不能为空")
 
+        # ★ 借库申请必填：申请原因；预计归还日期(除非勾选长期借用/不限归还)
+        if not str(remark or '').strip():
+            raise ValueError("借库申请原因不能为空")
+        _has_indefinite = any(bool(item.get('is_indefinite')) for item in items)
+        _has_date = any(str(item.get('expected_return_time') or '').strip() for item in items)
+        if not _has_indefinite and not _has_date:
+            raise ValueError("请填写预计归还日期，或勾选长期借用(不限归还)")
+
         required_fields = ['name', 'spec_model', 'quantity']
         for idx, item in enumerate(items):
             missing_fields = [f for f in required_fields if f not in item or str(item.get(f) or '').strip() == '']