Add cascading delete to Gift and Guest relationships

Updated the `ondelete="CASCADE"` behavior for foreign key constraints in Alembic migrations and added cascading rules to related models in SQLAlchemy. These changes ensure proper cleanup of dependent records when a parent record is deleted.

backend/app/models/gift.py

@@ -68,10 +68,10 @@ class GiftItem(Base, UUIDMixin, TimestampMixin):
     category = relationship("GiftCategory", back_populates="gifts")
     reserved_by = relationship("Guest",
                                secondary="guest_gifts",
-                               back_populates="gifts")
+                               back_populates="gifts", cascade='all')
     purchase_history = relationship("GiftPurchase",
                                     back_populates="gift_item",
-                                    order_by="desc(GiftPurchase.purchased_at)")
+                                    order_by="desc(GiftPurchase.purchased_at)", cascade='all, delete-orphan')
 
     def __repr__(self):
         return f"<GiftItem {self.name} ({self.status.value})>"