Restrict UserId visibility and update query includes

The `UserId` property in `ReadEnvelopeQuery` was changed from `public` to `internal` to improve encapsulation. A new `Authorize` method was added to set the `UserId` property using the `with` expression.

In `ReadEnvelopeQueryHandler`, the LINQ query was updated to replace the inclusion of `Documents` with `EnvelopeReceivers` and their associated `Receiver`, reflecting a shift in the data being eagerly loaded to support updated functionality.

EnvelopeGenerator.Application/Envelopes/Queries/ReadEnvelopeQuery.cs

@@ -22,7 +22,7 @@ public record ReadEnvelopeQuery : EnvelopeQueryBase, IRequest<IEnumerable<Envelo
     /// <summary>
     /// Optionaler Benutzerfilter; wenn gesetzt, werden nur Umschläge des Benutzers geladen.
     /// </summary>
-    public int? UserId { get; init; }
+    internal int? UserId { get; init; }
 
     /// <summary>
     /// Setzt den Benutzerkontext für die Abfrage.
@@ -133,7 +133,7 @@ public class ReadEnvelopeQueryHandler : IRequestHandler<ReadEnvelopeQuery, IEnum
         }
 
         var envelopes = await query
-            .Include(e => e.Documents)
+            .Include(e => e.EnvelopeReceivers).ThenInclude(er => er.Receiver)
             .ToListAsync(cancel);
 
         return _mapper.Map<IEnumerable<EnvelopeDto>>(envelopes);