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/UpdateTask.cs

@@ -0,0 +1,37 @@
+using AutoMapper;
+using MediatR;
+using Mws.Application.Common;
+
+namespace Mws.Application.Tasks;
+
+public record UpdateTaskCommand(Guid UserId, Guid TaskId, UpdateTaskRequest Request) : IRequest<TaskDto>;
+
+public class UpdateTaskHandler(IUnitOfWork uow, IMapper mapper) : IRequestHandler<UpdateTaskCommand, TaskDto>
+{
+    public async Task<TaskDto> Handle(UpdateTaskCommand command, CancellationToken ct)
+    {
+        var task = await TaskAccess.GetTaskForUserAsync(uow, command.UserId, command.TaskId, ct);
+
+        var request = command.Request;
+        if (string.IsNullOrWhiteSpace(request.Title))
+        {
+            throw new BadRequestException("Task title is required");
+        }
+
+        if (request.AssigneeId.HasValue && !await uow.Projects.IsMemberAsync(task.ProjectId, request.AssigneeId.Value, ct))
+        {
+            throw new BadRequestException("Assignee must be a member of the project");
+        }
+
+        task.Title = request.Title.Trim();
+        task.Description = request.Description;
+        task.Status = request.Status;
+        task.Priority = request.Priority;
+        task.AssigneeId = request.AssigneeId;
+        task.DueDate = request.DueDate;
+        task.UpdatedAt = DateTime.UtcNow;
+
+        await uow.SaveChangesAsync(ct);
+        return await TaskAccess.GetDtoAsync(uow, mapper, task.Id, ct);
+    }
+}