Adds the MediatR package and wires AddMediatR in DI. Replaces
IAuthService/AuthService with LoginCommand/LoginHandler, the pattern
the remaining modules will follow.
The ASP.NET Core solution (mws.api/mws.application/mws.domain/
mws.infrastructure) existed only as untracked working files. Adding
it to version control, plus a .gitignore for build output and local
tooling directories, so the CQRS/mediator refactor plan has a real
git history to branch and diff against.
namdh