Extend the Data Model for Order Management (UC-020)

Problem Description

The ShopCatalog team is preparing UC-020 "Place Order". The entity model at docs/entity_model.md has been extended with two new entities, Customer and Order, alongside the existing Product.

The project is a .NET 10 Blazor application using EF Core with PostgreSQL. The Product entity is already mapped (Domain/Product.cs, Data/ProductConfiguration.cs) and registered in Data/AppDbContext.cs.

Output Specification

Bring the EF Core model in line with docs/entity_model.md:

1. Entity classes for Customer and Order under Domain/, honoring every data type, length/precision, default, and relationship in the entity model.
2. EF Core configurations for both entities under Data/, and whatever AppDbContext changes are needed so the new entities are part of the model.
3. Migration: this environment has no .NET SDK installed, so do not try to execute the EF Core CLI. Instead, finish by writing out the exact migration command the team should run, following this project's migration naming convention for use-case-driven schema changes.

Do not create any test files, and do not put database credentials anywhere in source code.