Changed `EnvelopeReceiverReadOnlyDto` and `EnvelopeReceiverReadOnlyUpdateDto` from records to classes, converting properties to standard get/set accessors and adding documentation comments. Introduced a default value for `ChangedWhen` in `EnvelopeReceiverReadOnlyUpdateDto`.
Modified `ReceiverCreateDto` to a class structure, updating properties to use init-only accessors and encapsulating SHA256 hash logic in a private field. Retained the `AddedWhen` property with its default value.
This commit introduces the `[ApiExplorerSettings(IgnoreApi = true)]` attribute to various DTO classes to exclude them from API documentation. The `using Microsoft.AspNetCore.Mvc;` directive has been added to several files to support ASP.NET Core MVC features.
Additionally, comments in `HistoryController.cs` have been reformatted for clarity, and `LocalizationController.cs` has been updated with standard API controller attributes. These changes improve code organization and maintain cleaner API documentation.
