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/TaskAccess.cs

@@ -0,0 +1,35 @@
+using AutoMapper;
+using Mws.Application.Common;
+using Mws.Domain.Tasks;
+
+namespace Mws.Application.Tasks;
+
+internal static class TaskAccess
+{
+    public static async Task<TaskDto> GetDtoAsync(IUnitOfWork uow, IMapper mapper, Guid id, CancellationToken ct)
+    {
+        var task = await uow.Tasks.GetWithAssigneeAsync(id, ct)
+            ?? throw new NotFoundException("Task not found");
+        return mapper.Map<TaskDto>(task);
+    }
+
+    public static async Task<TaskItem> GetTaskForUserAsync(IUnitOfWork uow, Guid userId, Guid taskId, CancellationToken ct)
+    {
+        var task = await uow.Tasks.GetByIdAsync(taskId, ct)
+            ?? throw new NotFoundException("Task not found");
+
+        if (!await uow.Projects.IsMemberAsync(task.ProjectId, userId, ct))
+        {
+            throw new NotFoundException("Task not found");
+        }
+        return task;
+    }
+
+    public static async Task EnsureMemberAccessAsync(IUnitOfWork uow, Guid userId, Guid projectId, CancellationToken ct)
+    {
+        if (!await uow.Projects.IsMemberAsync(projectId, userId, ct))
+        {
+            throw new NotFoundException("Project not found");
+        }
+    }
+}