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/CreateTask.cs

@@ -0,0 +1,49 @@
+using AutoMapper;
+using MediatR;
+using Mws.Application.Common;
+using Mws.Domain.Tasks;
+using TaskPriority = Mws.Domain.Tasks.TaskPriority;
+using TaskStatus = Mws.Domain.Tasks.TaskStatus;
+
+namespace Mws.Application.Tasks;
+
+public record CreateTaskCommand(Guid UserId, Guid ProjectId, CreateTaskRequest Request) : IRequest<TaskDto>;
+
+public class CreateTaskHandler(IUnitOfWork uow, IMapper mapper) : IRequestHandler<CreateTaskCommand, TaskDto>
+{
+    public async Task<TaskDto> Handle(CreateTaskCommand command, CancellationToken ct)
+    {
+        await TaskAccess.EnsureMemberAccessAsync(uow, command.UserId, command.ProjectId, 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(command.ProjectId, request.AssigneeId.Value, ct))
+        {
+            throw new BadRequestException("Assignee must be a member of the project");
+        }
+
+        var now = DateTime.UtcNow;
+        var task = new TaskItem
+        {
+            Id = Guid.NewGuid(),
+            ProjectId = command.ProjectId,
+            Title = request.Title.Trim(),
+            Description = request.Description,
+            Status = request.Status ?? TaskStatus.Todo,
+            Priority = request.Priority ?? TaskPriority.Medium,
+            AssigneeId = request.AssigneeId,
+            DueDate = request.DueDate,
+            CreatedBy = command.UserId,
+            CreatedAt = now,
+            UpdatedAt = now,
+        };
+
+        uow.Tasks.Add(task);
+        await uow.SaveChangesAsync(ct);
+        return await TaskAccess.GetDtoAsync(uow, mapper, task.Id, ct);
+    }
+}