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.Tests/Repository/TestConnectionFactory.cs

@@ -0,0 +1,11 @@
+using System.Data.Common;
+using Infrastructure.Sql;
+
+namespace Features.UserManagement.Tests.Repository;
+
+internal class TestConnectionFactory(DbConnection conn) : ISqlConnectionFactory
+{
+    private readonly DbConnection _conn = conn;
+
+    public DbConnection CreateConnection() => _conn;
+}