Add Forecast model to ORM layer

- the model handles the caching of demand forecasting results
- include the database migration script

src/urban_meal_delivery/db/pixels.py

@@ -39,6 +39,7 @@ class Pixel(meta.Base):
     # Relationships
     grid = orm.relationship('Grid', back_populates='pixels')
     addresses = orm.relationship('AddressPixelAssociation', back_populates='pixel')
+    forecasts = orm.relationship('Forecast', back_populates='pixel')
 
     def __repr__(self) -> str:
         """Non-literal text representation."""