fix(models): remove relationship declarations that require ForeignKey constraints

The project does not use SQLAlchemy ORM-level relationships or ForeignKey
constraints. All joins are done manually in CRUD queries. The relationship()
declarations on BizProductionWorkOrder.tasks and BizProductionTask.steps
caused mapper initialization failures because the child table columns lack
ForeignKey definitions.

backend/app/models/models.py

@@ -3,7 +3,6 @@ from datetime import datetime
 import enum
 
 from sqlalchemy import Column, String, DateTime, Integer, Text, CHAR, SmallInteger, Numeric
-from sqlalchemy.orm import relationship
 
 from app.core.database import Base
 
@@ -2174,7 +2173,6 @@ class BizProductionWorkOrder(Base):
     update_time = Column(DateTime, nullable=False, default=datetime.now, onupdate=datetime.now, comment="更新时间")
     del_flag = Column(SmallInteger, nullable=False, default=0, comment="删除标记")
 
-    tasks = relationship("BizProductionTask", backref="work_order", lazy="dynamic")
 
 
 class BizProductionTask(Base):
@@ -2202,7 +2200,6 @@ class BizProductionTask(Base):
     update_time = Column(DateTime, nullable=False, default=datetime.now, onupdate=datetime.now, comment="更新时间")
     del_flag = Column(SmallInteger, nullable=False, default=0, comment="删除标记")
 
-    steps = relationship("BizProductionTaskStep", backref="task", lazy="dynamic")
 
 
 class BizProductionTaskInput(Base):