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/Queries/GetUserByIdHandlerTests.cs

@@ -0,0 +1,41 @@
+using Domain.Entities;
+using Domain.Exceptions;
+using FluentAssertions;
+using Features.UserManagement.Queries.GetUserById;
+using Features.UserManagement.Repository;
+using Moq;
+
+namespace Features.UserManagement.Tests.Queries;
+
+public class GetUserByIdHandlerTests
+{
+    private readonly Mock<IUserAccountRepository> _repoMock = new();
+    private readonly GetUserByIdHandler _handler;
+
+    public GetUserByIdHandlerTests()
+    {
+        _handler = new GetUserByIdHandler(_repoMock.Object);
+    }
+
+    [Fact]
+    public async Task Handle_ReturnsUser_WhenFound()
+    {
+        var user = new UserAccount { UserAccountId = Guid.NewGuid(), Username = "test" };
+        _repoMock.Setup(r => r.GetByIdAsync(user.UserAccountId)).ReturnsAsync(user);
+
+        var result = await _handler.Handle(new GetUserByIdQuery(user.UserAccountId), CancellationToken.None);
+
+        result.Should().Be(user);
+    }
+
+    [Fact]
+    public async Task Handle_Throws_WhenNotFound()
+    {
+        var id = Guid.NewGuid();
+        _repoMock.Setup(r => r.GetByIdAsync(id)).ReturnsAsync((UserAccount?)null);
+
+        var act = async () => await _handler.Handle(new GetUserByIdQuery(id), CancellationToken.None);
+
+        await act.Should().ThrowAsync<NotFoundException>();
+    }
+}