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/DependencyInjection/FeaturesUserManagementServiceCollectionExtensions.cs

@@ -0,0 +1,16 @@
+using Features.UserManagement.Repository;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Features.UserManagement.DependencyInjection;
+
+/// <summary>
+/// Registers the services owned by the UserManagement feature slice.
+/// </summary>
+public static class FeaturesUserManagementServiceCollectionExtensions
+{
+    public static IServiceCollection AddFeaturesUserManagement(this IServiceCollection services)
+    {
+        services.AddScoped<IUserAccountRepository, UserAccountRepository>();
+        return services;
+    }
+}