Migrate UserManagement to a vertical slice (Features.UserManagement)

Same pattern as the Breweries migration: Service.UserManagement and the
UserAccount repository fold into Features.UserManagement, with MediatR
queries replacing UserService. UpdateUserCommand/Handler carry forward
IUserService.UpdateAsync as-is (it has no HTTP route today, and adding one
is a separate decision from this migration). Adds handler/repository test
coverage that didn't exist before, since Service.UserManagement had no
test project.

web/backend/Features/Features.UserManagement/Commands/UpdateUser/UpdateUserHandler.cs

@@ -0,0 +1,15 @@
+using Features.UserManagement.Repository;
+using MediatR;
+
+namespace Features.UserManagement.Commands.UpdateUser;
+
+/// <summary>
+/// Handles <see cref="UpdateUserCommand"/> by persisting changes to an existing user account.
+/// </summary>
+/// <param name="repository">Repository used to persist the updated user account.</param>
+public class UpdateUserHandler(IUserAccountRepository repository)
+    : IRequestHandler<UpdateUserCommand>
+{
+    public Task Handle(UpdateUserCommand request, CancellationToken cancellationToken) =>
+        repository.UpdateAsync(request.UserAccount);
+}