Commit MWS backend source tree

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.

mws.application/Common/Exceptions.cs

@@ -0,0 +1,9 @@
+namespace Mws.Application.Common;
+
+public class NotFoundException(string message) : Exception(message);
+
+public class ForbiddenException(string message) : Exception(message);
+
+public class UnauthorizedException(string message) : Exception(message);
+
+public class BadRequestException(string message) : Exception(message);