Convert Tasks module to MediatR commands/queries

Replaces ITaskService/TaskService, completing the CQRS/mediator
refactor. All 9 old service interfaces are now gone except
IPermissionService, ITokenService, and IPasswordHasher, which stay
plain injected services by design (see spec).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mws.application/Tasks/Commands/DeleteTask.cs

@@ -0,0 +1,16 @@
+using MediatR;
+using Mws.Application.Common;
+
+namespace Mws.Application.Tasks;
+
+public record DeleteTaskCommand(Guid UserId, Guid TaskId) : IRequest;
+
+public class DeleteTaskHandler(IUnitOfWork uow) : IRequestHandler<DeleteTaskCommand>
+{
+    public async Task Handle(DeleteTaskCommand command, CancellationToken ct)
+    {
+        var task = await TaskAccess.GetTaskForUserAsync(uow, command.UserId, command.TaskId, ct);
+        uow.Tasks.Remove(task);
+        await uow.SaveChangesAsync(ct);
+    }
+}