add updated_at fields

backend/models.py

@@ -40,6 +40,7 @@ class Shop(Base):
     city = Column(String, nullable=False)
     address = Column(String, nullable=True)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())
     
     # Relationships
     shopping_events = relationship("ShoppingEvent", back_populates="shop")
@@ -53,6 +54,7 @@ class ShoppingEvent(Base):
     total_amount = Column(Float, nullable=True)  # Total cost of the shopping event
     notes = Column(String, nullable=True)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())
     
     # Relationships
     shop = relationship("Shop", back_populates="shopping_events")