Add multi-tenancy to an existing ASP.NET Core application

Checked against Tenantry 0.5, .NET 10, EF Core 10 and PostgreSQL 16. Most applications do not start multi-tenant. They gain a second customer, then a third, and every query gains a WHERE OrganisationId = …. One query without it shows a customer another customer's data. This post adds tenant isolation to an application like that with Tenantry Core, the open-source part An ASP.NET Core API over EF Core and PostgreSQL.