时间格式，获取沉降数据接口limit

app/models/level_data.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String
+from sqlalchemy import Column, Integer, String, DateTime
 from ..core.database import Base
 
 class LevelData(Base):
@@ -9,4 +9,5 @@ class LevelData(Base):
     benchmarkids = Column(String(100), comment="工作基点名称序列")
     wsphigh = Column(String(100), comment="工作基点高程序列(m)")
     NYID = Column(String(100), nullable=False, comment="期数id", index=True)
-    createDate = Column(String(100), comment="上传时间")
+    createDate = Column(DateTime, comment="上传时间")
+    mtype = Column(String(100), comment="水准观测类型")