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/Queries/GetAllUsers/GetAllUsersHandler.cs

@@ -0,0 +1,16 @@
+using Domain.Entities;
+using Features.UserManagement.Repository;
+using MediatR;
+
+namespace Features.UserManagement.Queries.GetAllUsers;
+
+/// <summary>
+/// Handles <see cref="GetAllUsersQuery"/> by retrieving a paginated list of user accounts.
+/// </summary>
+/// <param name="repository">Repository used to query user account data.</param>
+public class GetAllUsersHandler(IUserAccountRepository repository)
+    : IRequestHandler<GetAllUsersQuery, IEnumerable<UserAccount>>
+{
+    public Task<IEnumerable<UserAccount>> Handle(GetAllUsersQuery request, CancellationToken cancellationToken) =>
+        repository.GetAllAsync(request.Limit, request.Offset);
+}