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.infrastructure/DependencyInjection.cs

@@ -5,7 +5,6 @@ using Mws.Application.Auth;
 using Mws.Application.Common;
 using Mws.Application.Permissions;
 using Mws.Application.Projects;
-using Mws.Application.Tasks;
 using Mws.Infrastructure.Authentication;
 using Mws.Infrastructure.Persistence;
 
@@ -38,7 +37,6 @@ public static class DependencyInjection
         services.AddScoped<ITokenService, JwtTokenService>();
 
         services.AddScoped<IPermissionService, PermissionService>();
-        services.AddScoped<ITaskService, TaskService>();
 
         return services;
     }