Create db relation between Cost and Category

[kaajoj]

Uncomment in Cost.cs:
// public Category Category { get; set; }

https://sqlite.org/foreignkeys.html
maybe use raw sql to add fk

https://dev.to/icebeam7/foreign-keys-in-local-databases-with-sqlite-net-and-net-maui-22a1

other solution:
move your DbContext code into a .NET Standard class library and use a dummy .NET Core console app with the tools.
cd MyDbContextProject
dotnet ef migrations add MyMigration --startup-project ..\MyDummyProject