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/Queries/GetUserById/GetUserByIdQuery.cs

@@ -0,0 +1,9 @@
+using Domain.Entities;
+using MediatR;
+
+namespace Features.UserManagement.Queries.GetUserById;
+
+/// <summary>
+/// Retrieves a single user account by its unique identifier.
+/// </summary>
+public record GetUserByIdQuery(Guid UserAccountId) : IRequest<UserAccount>;