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/Queries/GetDocument.cs

@@ -0,0 +1,16 @@
+using AutoMapper;
+using MediatR;
+using Mws.Application.Common;
+
+namespace Mws.Application.Documents;
+
+public record GetDocumentQuery(Guid UserId, Guid DocumentId) : IRequest<DocumentDto>;
+
+public class GetDocumentHandler(IUnitOfWork uow, IMapper mapper) : IRequestHandler<GetDocumentQuery, DocumentDto>
+{
+    public async Task<DocumentDto> Handle(GetDocumentQuery query, CancellationToken ct)
+    {
+        var doc = await DocumentAccess.GetDocumentForUserAsync(uow, query.UserId, query.DocumentId, ct);
+        return mapper.Map<DocumentDto>(doc);
+    }
+}