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/GetAllUsersHandlerTests.cs

@@ -0,0 +1,23 @@
+using Domain.Entities;
+using FluentAssertions;
+using Features.UserManagement.Queries.GetAllUsers;
+using Features.UserManagement.Repository;
+using Moq;
+
+namespace Features.UserManagement.Tests.Queries;
+
+public class GetAllUsersHandlerTests
+{
+    [Fact]
+    public async Task Handle_PassesLimitAndOffset_ToRepository()
+    {
+        var repoMock = new Mock<IUserAccountRepository>();
+        var handler = new GetAllUsersHandler(repoMock.Object);
+        repoMock.Setup(r => r.GetAllAsync(10, 5)).ReturnsAsync(Array.Empty<UserAccount>());
+
+        var result = await handler.Handle(new GetAllUsersQuery(10, 5), CancellationToken.None);
+
+        result.Should().BeEmpty();
+        repoMock.Verify(r => r.GetAllAsync(10, 5), Times.Once);
+    }
+}