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/Commands/UpdateUserHandlerTests.cs

@@ -0,0 +1,22 @@
+using Domain.Entities;
+using Features.UserManagement.Commands.UpdateUser;
+using Features.UserManagement.Repository;
+using Moq;
+
+namespace Features.UserManagement.Tests.Commands;
+
+public class UpdateUserHandlerTests
+{
+    [Fact]
+    public async Task Handle_DelegatesToRepository()
+    {
+        var repoMock = new Mock<IUserAccountRepository>();
+        var handler = new UpdateUserHandler(repoMock.Object);
+        var user = new UserAccount { UserAccountId = Guid.NewGuid() };
+        repoMock.Setup(r => r.UpdateAsync(user)).Returns(Task.CompletedTask);
+
+        await handler.Handle(new UpdateUserCommand(user), CancellationToken.None);
+
+        repoMock.Verify(r => r.UpdateAsync(user), Times.Once);
+    }
+}