Convert Documents module to MediatR commands/queries

Replaces IDocumentService/DocumentService. The permission-check and
descendant-deletion logic moves to a shared DocumentAccess static
helper used by all seven handlers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mws.infrastructure/DependencyInjection.cs

@@ -3,7 +3,6 @@ using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Mws.Application.Auth;
 using Mws.Application.Common;
-using Mws.Application.Documents;
 using Mws.Application.Permissions;
 using Mws.Application.Projects;
 using Mws.Application.Tasks;
@@ -39,7 +38,6 @@ public static class DependencyInjection
         services.AddScoped<ITokenService, JwtTokenService>();
 
         services.AddScoped<IPermissionService, PermissionService>();
-        services.AddScoped<IDocumentService, DocumentService>();
         services.AddScoped<ITaskService, TaskService>();
 
         return services;