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.application/Documents/Commands/DeleteDocument.cs

@@ -0,0 +1,20 @@
+using MediatR;
+using Mws.Application.Common;
+using Mws.Application.Permissions;
+
+namespace Mws.Application.Documents;
+
+public record DeleteDocumentCommand(Guid UserId, Guid DocumentId) : IRequest;
+
+public class DeleteDocumentHandler(IUnitOfWork uow) : IRequestHandler<DeleteDocumentCommand>
+{
+    public async Task Handle(DeleteDocumentCommand command, CancellationToken ct)
+    {
+        var doc = await DocumentAccess.GetDocumentForUserAsync(uow, command.UserId, command.DocumentId, ct);
+        await DocumentAccess.EnsureDocumentPermissionAsync(uow, command.UserId, doc.ProjectId, PermissionAction.Delete, ct);
+
+        await DocumentAccess.DeleteDescendantsAsync(uow, command.DocumentId, ct);
+        uow.Documents.Remove(doc);
+        await uow.SaveChangesAsync(ct);
+    }
+}