Refactor: Replace Signatures with DocReceiverElements

Updated the codebase to replace the `Signature` record with the new `DocReceiverElementCreateDto` record for better alignment with the domain model.

- Updated `EnvelopeReceiverController` to use `DocReceiverElements` instead of `Signatures` when iterating over `Receivers`.
- Replaced the `Signature` record with `DocReceiverElementCreateDto` in `CreateEnvelopeReceiverCommand`.
- Updated `ReceiverGetOrCreateCommand` to use a `DocReceiverElements` property instead of `Signatures`.

These changes ensure consistency and reflect a shift in how document-related data is represented.

EnvelopeGenerator.Application/EnvelopeReceivers/Commands/CreateEnvelopeReceiverCommand.cs

@@ -29,7 +29,7 @@ public record CreateEnvelopeReceiverCommand : CreateEnvelopeCommand, IRequest<Cr
 /// <param name="X">X-Position</param>
 /// <param name="Y">Y-Position</param>
 /// <param name="Page">Seite, auf der sie sich befindet</param>
-public record Signature([Required] double X, [Required] double Y, [Required] int Page);
+public record DocReceiverElementCreateDto([Required] double X, [Required] double Y, [Required] int Page);
 
 /// <summary>
 /// DTO für Empfänger, die erstellt oder abgerufen werden sollen.
@@ -41,7 +41,7 @@ public class ReceiverGetOrCreateCommand
     /// Unterschriften auf Dokumenten.
     /// </summary>
     [Required]
-    public List<Signature> Signatures { get; init; } = new();
+    public List<DocReceiverElementCreateDto> DocReceiverElements { get; init; } = new();
 
     /// <summary>
     /// Der Name, mit dem der Empfänger angesprochen werden soll.