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/Queries/SearchTasks.cs

@@ -0,0 +1,17 @@
+using AutoMapper;
+using MediatR;
+using Mws.Application.Common;
+
+namespace Mws.Application.Tasks;
+
+public record SearchTasksQuery(Guid UserId, string Term) : IRequest<List<TaskDto>>;
+
+public class SearchTasksHandler(IUnitOfWork uow, IMapper mapper) : IRequestHandler<SearchTasksQuery, List<TaskDto>>
+{
+    public async Task<List<TaskDto>> Handle(SearchTasksQuery query, CancellationToken ct)
+    {
+        var projectIds = await uow.Projects.GetProjectIdsForUserAsync(query.UserId, ct);
+        var tasks = await uow.Tasks.SearchWithAssigneeAsync(projectIds, query.Term, 20, ct);
+        return mapper.Map<List<TaskDto>>(tasks);
+    }
+}